docs/develop/develop-images/dockerfile_best-practices.md

---
description: Hints, tips and guidelines for writing clean, reliable Dockerfiles
keywords: parent image, images, dockerfile, best practices, hub, official image
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title: Best practices for writing Dockerfiles
---

This topic covers recommended best practices and methods for building
efficient images.

Docker builds images automatically by reading the instructions from a
Dockerfile -- a text file that contains all commands, in order, needed to
build a given image. A Dockerfile adheres to a specific format and set of
instructions which you can find at [Dockerfile reference](../../engine/reference/builder.md).

A Docker image consists of read-only layers each of which represents a
Dockerfile instruction. The layers are stacked and each one is a delta of the
changes from the previous layer. The following is the contents of an example Dockerfile:

```dockerfile
# syntax=docker/dockerfile:1

FROM ubuntu:22.04
COPY . /app
RUN make /app
CMD python /app/app.py
```

Each instruction creates one layer:

- `FROM` creates a layer from the `ubuntu:22.04` Docker image.
- `COPY` adds files from your Docker client's current directory.
- `RUN` builds your application with `make`.
- `CMD` specifies what command to run within the container.

When you run an image and generate a container, you add a new writable layer, also called the container layer, on top of the underlying layers. All changes made to
the running container, such as writing new files, modifying existing files, and
deleting files, are written to this writable container layer.

For more on image layers and how Docker builds and stores images, see
[About storage drivers](../../storage/storagedriver/index.md).

## General guidelines and recommendations

### Create ephemeral containers

The image defined by your Dockerfile should generate containers that are as
ephemeral as possible. Ephemeral means that the container can be stopped
and destroyed, then rebuilt and replaced with an absolute minimum set up and
configuration.

Refer to [Processes](https://12factor.net/processes) under _The Twelve-factor App_
methodology to get a feel for the motivations of running containers in such a
stateless fashion.

### Understand build context

See [Build context](../../build/building/context.md) for more information.

### Pipe Dockerfile through stdin

Docker has the ability to build images by piping a Dockerfile through stdin
with a local or remote build context. Piping a Dockerfile through stdin
can be useful to perform one-off builds without writing a Dockerfile to disk,
or in situations where the Dockerfile is generated, and should not persist
afterwards.

> **Note**
>
> The examples in the following sections use [here documents](https://tldp.org/LDP/abs/html/here-docs.html)
> for convenience, but any method to provide the Dockerfile on stdin can be
> used.
> 
> For example, the following commands are equivalent: 
> 
> ```bash
> echo -e 'FROM busybox\nRUN echo "hello world"' | docker build -
> ```
> 
> ```bash
> docker build -<<EOF
> FROM busybox
> RUN echo "hello world"
> EOF
> ```
> 
> You can substitute the examples with your preferred approach, or the approach
> that best fits your use case.


#### Build an image using a Dockerfile from stdin, without sending build context

Use this syntax to build an image using a Dockerfile from stdin, without
sending additional files as build context. The hyphen (`-`) takes the position
of the `PATH`, and instructs Docker to read the build context, which only
contains a Dockerfile, from stdin instead of a directory:

```bash
docker build [OPTIONS] -
```

The following example builds an image using a Dockerfile that is passed through
stdin. No files are sent as build context to the daemon.

```bash
docker build -t myimage:latest -<<EOF
FROM busybox
RUN echo "hello world"
EOF
```

Omitting the build context can be useful in situations where your Dockerfile
doesn't require files to be copied into the image, and improves the build speed,
as no files are sent to the daemon.

If you want to improve the build speed by excluding some files from the build
context, refer to [exclude with .dockerignore](#exclude-with-dockerignore).

> **Note**
>
> If you attempt to build an image using a Dockerfile from stdin, without sending a build context, then the build will fail if you use `COPY` or `ADD`.
> The following example illustrates this:
> 
> ```bash
> # create a directory to work in
> mkdir example
> cd example
> 
> # create an example file
> touch somefile.txt
> 
> docker build -t myimage:latest -<<EOF
> FROM busybox
> COPY somefile.txt ./
> RUN cat /somefile.txt
> EOF
> 
> # observe that the build fails
> ...
> Step 2/3 : COPY somefile.txt ./
> COPY failed: stat /var/lib/docker/tmp/docker-builder249218248/somefile.txt: no such file or directory
> ```

#### Build from a local build context, using a Dockerfile from stdin

Use this syntax to build an image using files on your local filesystem, but using
a Dockerfile from stdin. The syntax uses the `-f` (or `--file`) option to
specify the Dockerfile to use, and it uses a hyphen (`-`) as filename to instruct
Docker to read the Dockerfile from stdin:

```bash
docker build [OPTIONS] -f- PATH
```

The example below uses the current directory (`.`) as the build context, and builds
an image using a Dockerfile that is passed through stdin using a [here
document](https://tldp.org/LDP/abs/html/here-docs.html).

```bash
# create a directory to work in
mkdir example
cd example

# create an example file
touch somefile.txt

# build an image using the current directory as context, and a Dockerfile passed through stdin
docker build -t myimage:latest -f- . <<EOF
FROM busybox
COPY somefile.txt ./
RUN cat /somefile.txt
EOF
```

#### Build from a remote build context, using a Dockerfile from stdin

Use this syntax to build an image using files from a remote Git repository, 
using a Dockerfile from stdin. The syntax uses the `-f` (or `--file`) option to
specify the Dockerfile to use, using a hyphen (`-`) as filename to instruct
Docker to read the Dockerfile from stdin:

```bash
docker build [OPTIONS] -f- PATH
```

This syntax can be useful in situations where you want to build an image from a
repository that doesn't contain a Dockerfile, or if you want to build with a custom
Dockerfile, without maintaining your own fork of the repository.

The example below builds an image using a Dockerfile from stdin, and adds
the `hello.c` file from the [hello-world](https://github.com/docker-library/hello-world) repository on GitHub.

```bash
docker build -t myimage:latest -f- https://github.com/docker-library/hello-world.git <<EOF
FROM busybox
COPY hello.c ./
EOF
```

> **Note**
>
> When building an image using a remote Git repository as the build context, Docker
> performs a `git clone` of the repository on the local machine, and sends
> those files as the build context to the daemon. This feature requires you to
> install Git on the host where you run the `docker build` command.

### Exclude with .dockerignore

To exclude files not relevant to the build, without restructuring your source
repository, use a `.dockerignore` file. This file supports exclusion patterns
similar to `.gitignore` files. For information on creating one, see 
[.dockerignore file](../../engine/reference/builder.md#dockerignore-file).

### Use multi-stage builds

[Multi-stage builds](../../build/building/multi-stage.md) allow you to
drastically reduce the size of your final image, without struggling to reduce
the number of intermediate layers and files.

Because an image is built during the final stage of the build process, you can
minimize image layers by [leveraging build cache](#leverage-build-cache).

For example, if your build contains several layers and you want to ensure the build cache is reusable, you can order them from the less frequently changed to the more frequently changed. The following list is an example of the order of instructions:

1. Install tools you need to build your application

2. Install or update library dependencies

3. Generate your application

A Dockerfile for a Go application could look like:

```dockerfile
# syntax=docker/dockerfile:1
FROM golang:{{site.example_go_version}}-alpine AS build

# Install tools required for project
# Run `docker build --no-cache .` to update dependencies
RUN apk add --no-cache git

# List project dependencies with go.mod and go.sum
# These layers are only re-built when Gopkg files are updated
WORKDIR /go/src/project/
COPY go.mod go.sum /go/src/project/
# Install library dependencies
RUN go mod download

# Copy the entire project and build it
# This layer is rebuilt when a file changes in the project directory
COPY . /go/src/project/
RUN go build -o /bin/project

# This results in a single layer image
FROM scratch
COPY --from=build /bin/project /bin/project
ENTRYPOINT ["/bin/project"]
CMD ["--help"]
```

### Don't install unnecessary packages

Avoid installing extra or unnecessary packages just because they might be nice to have. For example, you don’t need to include a text editor in a database image.

When you avoid installing extra or unnecessary packages, your images will have reduced complexity, reduced dependencies, reduced file sizes, and reduced build times.

### Decouple applications

Each container should have only one concern. Decoupling applications into
multiple containers makes it easier to scale horizontally and reuse containers.
For instance, a web application stack might consist of three separate
containers, each with its own unique image, to manage the web application,
database, and an in-memory cache in a decoupled manner.

Limiting each container to one process is a good rule of thumb, but it's not a
hard and fast rule. For example, not only can containers be
[spawned with an init process](../../engine/reference/run.md#specify-an-init-process),
some programs might spawn additional processes of their own accord. For
instance, [Celery](https://docs.celeryproject.org/) can spawn multiple worker
processes, and [Apache](https://httpd.apache.org/) can create one process per
request.

Use your best judgment to keep containers as clean and modular as possible. If
containers depend on each other, you can use [Docker container networks](../../network/index.md)
to ensure that these containers can communicate.

### Minimize the number of layers

In older versions of Docker, it was important that you minimized the number of
layers in your images to ensure they were performant. The following features
were added to reduce this limitation:

- Only the instructions `RUN`, `COPY`, and `ADD` create layers. Other instructions
  create temporary intermediate images, and don't increase the size of the build.

- Where possible, use [multi-stage builds](../../build/building/multi-stage.md),
  and only copy the artifacts you need into the final image. This allows you to
  include tools and debug information in your intermediate build stages without
  increasing the size of the final image.

### Sort multi-line arguments

Whenever possible, sort multi-line arguments alphanumerically to make maintenance easier. 
This helps to avoid duplication of packages and make the
list much easier to update. This also makes PRs a lot easier to read and
review. Adding a space before a backslash (`\`) helps as well.

Here’s an example from the [buildpack-deps image](https://github.com/docker-library/buildpack-deps):

```dockerfile
RUN apt-get update && apt-get install -y \
  bzr \
  cvs \
  git \
  mercurial \
  subversion \
  && rm -rf /var/lib/apt/lists/*
```

### Leverage build cache

When building an image, Docker steps through the instructions in your
Dockerfile, executing each in the order specified. As each instruction is
examined, Docker looks for an existing image in its cache,
rather than creating a new, duplicate image.

If you don't want to use the cache at all, you can use the `--no-cache=true`
option on the `docker build` command. However, if you do let Docker use its
cache, it's important to understand when it can, and can't, find a matching
image. The basic rules that Docker follows are outlined below:

- Starting with a parent image that's already in the cache, the next
  instruction is compared against all child images derived from that base
  image to see if one of them was built using the exact same instruction. If
  not, the cache is invalidated.

- In most cases, simply comparing the instruction in the Dockerfile with one
  of the child images is sufficient. However, certain instructions require more
  examination and explanation.

- For the `ADD` and `COPY` instructions, the contents of each file
  in the image are examined and a checksum is calculated for each file.
  The last-modified and last-accessed times of each file aren't considered in
  these checksums. During the cache lookup, the checksum is compared against the
  checksum in the existing images. If anything has changed in any file, such
  as the contents and metadata, then the cache is invalidated.

- Aside from the `ADD` and `COPY` commands, cache checking doesn't look at the
  files in the container to determine a cache match. For example, when processing
  a `RUN apt-get -y update` command the files updated in the container
  aren't examined to determine if a cache hit exists. In that case just
  the command string itself is used to find a match.

Once the cache is invalidated, all subsequent Dockerfile commands generate new
images and the cache isn't used.

## Dockerfile instructions

These recommendations are designed to help you create an efficient and
maintainable Dockerfile.

### FROM

Whenever possible, use current official images as the basis for your
images. Docker recommends the [Alpine image](https://hub.docker.com/_/alpine/) as it
is tightly controlled and small in size (currently under 6 MB), while still
being a full Linux distribution.

For more information about the `FROM` instruction, see [Dockerfile reference for the FROM instruction](../../engine/reference/builder.md#from).

### LABEL

You can add labels to your image to help organize images by project, record
licensing information, to aid in automation, or for other reasons. For each
label, add a line beginning with `LABEL` with one or more key-value pairs.
The following examples show the different acceptable formats. Explanatory comments are included inline.

Strings with spaces must be quoted or the spaces must be escaped. Inner
quote characters (`"`), must also be escaped. For example:

```dockerfile
# Set one or more individual labels
LABEL com.example.version="0.0.1-beta"
LABEL vendor1="ACME Incorporated"
LABEL vendor2=ZENITH\ Incorporated
LABEL com.example.release-date="2015-02-12"
LABEL com.example.version.is-production=""
```

An image can have more than one label. Prior to Docker 1.10, it was recommended
to combine all labels into a single `LABEL` instruction, to prevent extra layers
from being created. This is no longer necessary, but combining labels is still
supported. For example:

```dockerfile
# Set multiple labels on one line
LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"
```

The above example can also be written as:

```dockerfile
# Set multiple labels at once, using line-continuation characters to break long lines
LABEL vendor=ACME\ Incorporated \
      com.example.is-beta= \
      com.example.is-production="" \
      com.example.version="0.0.1-beta" \
      com.example.release-date="2015-02-12"
```

See [Understanding object labels](../../config/labels-custom-metadata.md)
for guidelines about acceptable label keys and values. For information about
querying labels, refer to the items related to filtering in
[Managing labels on objects](../../config/labels-custom-metadata.md#manage-labels-on-objects).
See also [LABEL](../../engine/reference/builder.md#label) in the Dockerfile reference.

### RUN

Split long or complex `RUN` statements on multiple lines separated with
backslashes to make your Dockerfile more readable, understandable, and
maintainable.

For more information about `RUN`, see [Dockerfile reference for the RUN instruction](../../engine/reference/builder.md#run).

#### apt-get

Probably the most common use case for `RUN` is an application of `apt-get`.
Because it installs packages, the `RUN apt-get` command has several counter-intuitive behaviors to
look out for.

Always combine `RUN apt-get update` with `apt-get install` in the same `RUN`
statement. For example:

```dockerfile
RUN apt-get update && apt-get install -y \
    package-bar \
    package-baz \
    package-foo  \
    && rm -rf /var/lib/apt/lists/*
```

Using `apt-get update` alone in a `RUN` statement causes caching issues and
subsequent `apt-get install` instructions to fail. For example, this issue will occur in the following Dockerfile:

```dockerfile
# syntax=docker/dockerfile:1

FROM ubuntu:22.04
RUN apt-get update
RUN apt-get install -y curl
```

After building the image, all layers are in the Docker cache. Suppose you later
modify `apt-get install` by adding an extra package as shown in the following Dockerfile:

```dockerfile
# syntax=docker/dockerfile:1

FROM ubuntu:22.04
RUN apt-get update
RUN apt-get install -y curl nginx
```

Docker sees the initial and modified instructions as identical and reuses the
cache from previous steps. As a result the `apt-get update` isn't executed
because the build uses the cached version. Because the `apt-get update` isn't
run, your build can potentially get an outdated version of the `curl` and
`nginx` packages.

Using `RUN apt-get update && apt-get install -y` ensures your Dockerfile
installs the latest package versions with no further coding or manual
intervention. This technique is known as cache busting. You can also achieve
cache busting by specifying a package version. This is known as version pinning.
For example:

```dockerfile
RUN apt-get update && apt-get install -y \
    package-bar \
    package-baz \
    package-foo=1.3.*
```

Version pinning forces the build to retrieve a particular version regardless of
what’s in the cache. This technique can also reduce failures due to unanticipated changes
in required packages.

Below is a well-formed `RUN` instruction that demonstrates all the `apt-get`
recommendations.

```dockerfile
RUN apt-get update && apt-get install -y \
    aufs-tools \
    automake \
    build-essential \
    curl \
    dpkg-sig \
    libcap-dev \
    libsqlite3-dev \
    mercurial \
    reprepro \
    ruby1.9.1 \
    ruby1.9.1-dev \
    s3cmd=1.1.* \
 && rm -rf /var/lib/apt/lists/*
```

The `s3cmd` argument specifies a version `1.1.*`. If the image previously
used an older version, specifying the new one causes a cache bust of `apt-get
update` and ensures the installation of the new version. Listing packages on
each line can also prevent mistakes in package duplication.

In addition, when you clean up the apt cache by removing `/var/lib/apt/lists` it
reduces the image size, since the apt cache isn't stored in a layer. Since the
`RUN` statement starts with `apt-get update`, the package cache is always
refreshed prior to `apt-get install`.

Official Debian and Ubuntu images [automatically run `apt-get clean`](https://github.com/moby/moby/blob/03e2923e42446dbb830c654d0eec323a0b4ef02a/contrib/mkimage/debootstrap#L82-L105), so explicit invocation is not required.

#### Using pipes

Some `RUN` commands depend on the ability to pipe the output of one command into another, using the pipe character (`|`), as in the following example:

```dockerfile
RUN wget -O - https://some.site | wc -l > /number
```

Docker executes these commands using the `/bin/sh -c` interpreter, which only
evaluates the exit code of the last operation in the pipe to determine success.
In the example above, this build step succeeds and produces a new image so long
as the `wc -l` command succeeds, even if the `wget` command fails.

If you want the command to fail due to an error at any stage in the pipe,
prepend `set -o pipefail &&` to ensure that an unexpected error prevents the
build from inadvertently succeeding. For example:

```dockerfile
RUN set -o pipefail && wget -O - https://some.site | wc -l > /number
```

> **Note**
>
> Not all shells support the `-o pipefail` option.
>
> In cases such as the `dash` shell on
> Debian-based images, consider using the _exec_ form of `RUN` to explicitly
> choose a shell that does support the `pipefail` option. For example:
>
> ```dockerfile
> RUN ["/bin/bash", "-c", "set -o pipefail && wget -O - https://some.site | wc -l > /number"]
> ```

### CMD

The `CMD` instruction should be used to run the software contained in your
image, along with any arguments. `CMD` should almost always be used in the form
of `CMD ["executable", "param1", "param2"]`. Thus, if the image is for a
service, such as Apache and Rails, you would run something like `CMD
["apache2","-DFOREGROUND"]`. Indeed, this form of the instruction is recommended
for any service-based image.

In most other cases, `CMD` should be given an interactive shell, such as bash,
python and perl. For example, `CMD ["perl", "-de0"]`, `CMD ["python"]`, or `CMD
["php", "-a"]`. Using this form means that when you execute something like
`docker run -it python`, you’ll get dropped into a usable shell, ready to go.
`CMD` should rarely be used in the manner of `CMD ["param", "param"]` in
conjunction with [`ENTRYPOINT`](../../engine/reference/builder.md#entrypoint), unless
you and your expected users are already quite familiar with how `ENTRYPOINT`
works.

For more information about `CMD`, see [Dockerfile reference for the CMD instruction](../../engine/reference/builder.md#cmd).

### EXPOSE

The `EXPOSE` instruction indicates the ports on which a container listens
for connections. Consequently, you should use the common, traditional port for
your application. For example, an image containing the Apache web server would
use `EXPOSE 80`, while an image containing MongoDB would use `EXPOSE 27017` and
so on.

For external access, your users can execute `docker run` with a flag indicating
how to map the specified port to the port of their choice.
For container linking, Docker provides environment variables for the path from
the recipient container back to the source (ie, `MYSQL_PORT_3306_TCP`).

For more information about `EXPOSE`, see [Dockerfile reference for the EXPOSE instruction](../../engine/reference/builder.md#expose).

### ENV

To make new software easier to run, you can use `ENV` to update the
`PATH` environment variable for the software your container installs. For
example, `ENV PATH=/usr/local/nginx/bin:$PATH` ensures that `CMD ["nginx"]`
just works.

The `ENV` instruction is also useful for providing the required environment
variables specific to services you wish to containerize, such as Postgres’s
`PGDATA`.

Lastly, `ENV` can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example:

```dockerfile
ENV PG_MAJOR=9.3
ENV PG_VERSION=9.3.4
RUN curl -SL https://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgres && …
ENV PATH=/usr/local/postgres-$PG_MAJOR/bin:$PATH
```

Similar to having constant variables in a program, as opposed to hard-coding
values, this approach lets you change a single `ENV` instruction to
automatically bump the version of the software in your container.

Each `ENV` line creates a new intermediate layer, just like `RUN` commands. This
means that even if you unset the environment variable in a future layer, it
still persists in this layer and its value can be dumped. You can test this by
creating a Dockerfile like the following, and then building it.

```dockerfile
# syntax=docker/dockerfile:1
FROM alpine
ENV ADMIN_USER="mark"
RUN echo $ADMIN_USER > ./mark
RUN unset ADMIN_USER
```

```console
$ docker run --rm test sh -c 'echo $ADMIN_USER'

mark
```

To prevent this, and really unset the environment variable, use a `RUN` command
with shell commands, to set, use, and unset the variable all in a single layer.
You can separate your commands with `;` or `&&`. If you use the second method,
and one of the commands fails, the `docker build` also fails. This is usually a
good idea. Using `\` as a line continuation character for Linux Dockerfiles
improves readability. You could also put all of the commands into a shell script
and have the `RUN` command just run that shell script.

```dockerfile
# syntax=docker/dockerfile:1
FROM alpine
RUN export ADMIN_USER="mark" \
    && echo $ADMIN_USER > ./mark \
    && unset ADMIN_USER
CMD sh
```

```console
$ docker run --rm test sh -c 'echo $ADMIN_USER'

```

For more information about `ENV`, see [Dockerfile reference for the ENV instruction](../../engine/reference/builder.md#env).

### ADD or COPY

Although `ADD` and `COPY` are functionally similar, generally speaking, `COPY`
is preferred. That’s because it’s more transparent than `ADD`. `COPY` only
supports the basic copying of local files into the container, while `ADD` has
some features (like local-only tar extraction and remote URL support) that are
not immediately obvious. Consequently, the best use for `ADD` is local tar file
auto-extraction into the image, as in `ADD rootfs.tar.xz /`.

If you have multiple Dockerfile steps that use different files from your
context, `COPY` them individually, rather than all at once. If
a specifically required file changes, then this ensures that
only that step's build cache is invalidated, forcing only that step to be run again.

For example:

```dockerfile
COPY requirements.txt /tmp/
RUN pip install --requirement /tmp/requirements.txt
COPY . /tmp/
```

Results in fewer cache invalidations for the `RUN` step, than if you put the
`COPY . /tmp/` before it.

Because image size matters, using `ADD` to fetch packages from remote URLs is
strongly discouraged; you should use `curl` or `wget` instead. That way you can
delete the files you no longer need after they've been extracted and you don't
have to add another layer in your image. For example, you should avoid doing
things like:

```dockerfile
ADD https://example.com/big.tar.xz /usr/src/things/
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all
```

And instead, do something like:

```dockerfile
RUN mkdir -p /usr/src/things \
    && curl -SL https://example.com/big.tar.xz \
    | tar -xJC /usr/src/things \
    && make -C /usr/src/things all
```

For other items, like files and directories, that don't require the tar
auto-extraction capability of `ADD`, you should always use `COPY`.

For more information about `ADD` or `COPY`, see the following:
- [Dockerfile reference for the ADD instruction](../../engine/reference/builder.md#add)
- [Dockerfile reference for the COPY instruction](../../engine/reference/builder.md#copy)


### ENTRYPOINT

The best use for `ENTRYPOINT` is to set the image's main command, allowing that
image to be run as though it was that command, and then use `CMD` as the
default flags.

The following is an example of an image for the command line tool `s3cmd`:

```dockerfile
ENTRYPOINT ["s3cmd"]
CMD ["--help"]
```

You can use the following command to run the image and show the command's help:

```console
$ docker run s3cmd
```

Or, you can use the right parameters to execute a command, like in the following example:

```console
$ docker run s3cmd ls s3://mybucket
```

This is useful because the image name can double as a reference to the binary as
shown in the command above.

The `ENTRYPOINT` instruction can also be used in combination with a helper
script, allowing it to function in a similar way to the command above, even
when starting the tool may require more than one step.

For example, the [Postgres Official Image](https://hub.docker.com/_/postgres/)
uses the following script as its `ENTRYPOINT`:

```bash
#!/bin/bash
set -e

if [ "$1" = 'postgres' ]; then
    chown -R postgres "$PGDATA"

    if [ -z "$(ls -A "$PGDATA")" ]; then
        gosu postgres initdb
    fi

    exec gosu postgres "$@"
fi

exec "$@"
```


This script uses [the `exec` Bash command](https://wiki.bash-hackers.org/commands/builtin/exec) so that the final running application becomes the container's PID 1. This allows the application to receive any Unix signals sent to the container. For more information, see the [`ENTRYPOINT` reference](../../engine/reference/builder.md#entrypoint).

In the following example, a helper script is copied into the container and run via `ENTRYPOINT` on
container start:

```dockerfile
COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["postgres"]
```

This script allows the user to interact with Postgres in several ways.

It can simply start Postgres:

```console
$ docker run postgres
```

Or, it can be used to run Postgres and pass parameters to the server:

```console
$ docker run postgres postgres --help
```

Lastly, it could also be used to start a totally different tool, such as Bash:

```console
$ docker run --rm -it postgres bash
```

For more information about `ENTRYPOINT`, see [Dockerfile reference for the ENTRYPOINT instruction](../../engine/reference/builder.md#entrypoint).

### VOLUME

The `VOLUME` instruction should be used to expose any database storage area,
configuration storage, or files and folders created by your Docker container. You
are strongly encouraged to use `VOLUME` for any combination of mutable or user-serviceable
parts of your image.

For more information about `VOLUME`, see [Dockerfile reference for the VOLUME instruction](../../engine/reference/builder.md#volume).

### USER

If a service can run without privileges, use `USER` to change to a non-root
user. Start by creating the user and group in the Dockerfile with something
like the following example:

```dockerfile
RUN groupadd -r postgres && useradd --no-log-init -r -g postgres postgres
```

> **Note**
>
> Consider an explicit UID/GID.
>
> Users and groups in an image are assigned a non-deterministic UID/GID in that
> the "next" UID/GID is assigned regardless of image rebuilds. So, if it’s
> critical, you should assign an explicit UID/GID.

> **Note**
>
> Due to an [unresolved bug](https://github.com/golang/go/issues/13548) in the
> Go archive/tar package's handling of sparse files, attempting to create a user
> with a significantly large UID inside a Docker container can lead to disk
> exhaustion because `/var/log/faillog` in the container layer is filled with
> NULL (\0) characters. A workaround is to pass the `--no-log-init` flag to
> useradd. The Debian/Ubuntu `adduser` wrapper does not support this flag.

Avoid installing or using `sudo` as it has unpredictable TTY and
signal-forwarding behavior that can cause problems. If you absolutely need
functionality similar to `sudo`, such as initializing the daemon as `root` but
running it as non-`root`, consider using [“gosu”](https://github.com/tianon/gosu).

Lastly, to reduce layers and complexity, avoid switching `USER` back and forth
frequently.

For more information about `USER`, see [Dockerfile reference for the USER instruction](../../engine/reference/builder.md#user).

### WORKDIR

For clarity and reliability, you should always use absolute paths for your
`WORKDIR`. Also, you should use `WORKDIR` instead of  proliferating instructions
like `RUN cd … && do-something`, which are hard to read, troubleshoot, and
maintain.

For more information about `WORKDIR`, see [Dockerfile reference for the WORKDIR instruction](../../engine/reference/builder.md#workdir).

### ONBUILD

An `ONBUILD` command executes after the current Dockerfile build completes.
`ONBUILD` executes in any child image derived `FROM` the current image. Think
of the `ONBUILD` command as an instruction that the parent Dockerfile gives
to the child Dockerfile.

A Docker build executes `ONBUILD` commands before any command in a child
Dockerfile.

`ONBUILD` is useful for images that are going to be built `FROM` a given
image. For example, you would use `ONBUILD` for a language stack image that
builds arbitrary user software written in that language within the
Dockerfile, as you can see in [Ruby’s `ONBUILD` variants](https://github.com/docker-library/ruby/blob/c43fef8a60cea31eb9e7d960a076d633cb62ba8d/2.4/jessie/onbuild/Dockerfile).

Images built with `ONBUILD` should get a separate tag. For example,
`ruby:1.9-onbuild` or `ruby:2.0-onbuild`.

Be careful when putting `ADD` or `COPY` in `ONBUILD`. The onbuild image
fails catastrophically if the new build's context is missing the resource being
added. Adding a separate tag, as recommended above, helps mitigate this by
allowing the Dockerfile author to make a choice.

For more information about `ONBUILD`, see [Dockerfile reference for the ONBUILD instruction](../../engine/reference/builder.md#onbuild).

## Examples of Docker Official Images

These Official Images have exemplary Dockerfiles:

* [Go](https://hub.docker.com/_/golang/)
* [Perl](https://hub.docker.com/_/perl/)
* [Hy](https://hub.docker.com/_/hylang/)
* [Ruby](https://hub.docker.com/_/ruby/)

## Additional resources:

* [Dockerfile Reference](../../engine/reference/builder.md)
* [More about Automated Builds](../../docker-hub/builds/index.md)
* [Guidelines for Creating Docker Official Images](../../docker-hub/official_images.md)
* [Best practices to containerize Node.js web applications with Docker](https://snyk.io/blog/10-best-practices-to-containerize-nodejs-web-applications-with-docker){:target="_blank" rel="noopener" class="_"}
* [More about Base Images](../../build/building/base-images.md)