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Refresh image best practices (#16603) 

* Refresh style for image best practices
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@ -10,17 +10,17 @@ redirect_from:
title: Best practices for writing Dockerfiles
---
This document covers recommended best practices and methods for building
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
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. Consider this `Dockerfile`:
changes from the previous layer. The following is the contents of an example Dockerfile:
```dockerfile
# syntax=docker/dockerfile:1
@ -37,20 +37,19 @@ Each instruction creates one layer:
- `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_
(the "container layer") on top of the underlying layers. All changes made to
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
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. By "ephemeral", we mean that the container can be stopped
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.
@ -60,18 +59,20 @@ stateless fashion.
### Understand build context
See [Build context](../../build/building/context.md) page for more information.
See [Build context](../../build/building/context.md) for more information.
### Pipe Dockerfile through `stdin`
### Pipe Dockerfile through stdin
Docker has the ability to build images by piping `Dockerfile` through `stdin`
with a _local or remote build context_. Piping a `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
or in situations where the Dockerfile is generated, and should not persist
afterwards.
> The examples in this section 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
> **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:
@ -93,17 +94,17 @@ afterwards.
#### 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
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:
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.
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
@ -112,15 +113,17 @@ RUN echo "hello world"
EOF
```
Omitting the build context can be useful in situations where your `Dockerfile`
does not require files to be copied into the image, and improves the build-speed,
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-
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**: Attempting to build a Dockerfile that uses `COPY` or `ADD` will fail
> if this syntax is used. The following example illustrates this:
> **Note**
>
> If you attempt build an image using a Dockerfile from stdin, without sending 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
@ -145,16 +148,16 @@ context, refer to [exclude with .dockerignore](#exclude-with-dockerignore).
#### 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, using a hyphen (`-`) as filename to instruct
Docker to read the `Dockerfile` from `stdin`:
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
an image using a Dockerfile that is passed through stdin using a [here
document](https://tldp.org/LDP/abs/html/here-docs.html).
```bash
@ -175,21 +178,21 @@ 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`:
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 does not contain a `Dockerfile`, or if you want to build with a custom
`Dockerfile`, without maintaining your own fork of the repository.
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" Git repository on GitHub](https://github.com/docker-library/hello-world).
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
@ -198,18 +201,18 @@ COPY hello.c ./
EOF
```
> **Under the hood**
> **Note**
>
> When building an image using a remote Git repository as build context, Docker
> performs a `git clone` of the repository on the local machine, and sends
> those files as build context to the daemon. This feature requires `git` to be
> installed on the host where you run the `docker build` command.
> those files as 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 the
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
@ -221,15 +224,13 @@ 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, you can order them from the
less frequently changed (to ensure the build cache is reusable) to the more
frequently changed:
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:
* Install tools you need to build your application
1. Install tools you need to build your application
* Install or update library dependencies
2. Install or update library dependencies
* Generate your application
3. Generate your application
A Dockerfile for a Go application could look like:
@ -263,9 +264,9 @@ CMD ["--help"]
### Don't install unnecessary packages
To reduce complexity, dependencies, file sizes, and build times, 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.
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, you images will have reduced complexity, reduced dependencies, reduced file sizes, and reduced build times.
### Decouple applications
@ -275,7 +276,7 @@ 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 is not a
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
@ -294,7 +295,7 @@ layers in your images to ensure they were performant. The following features
were added to reduce this limitation:
- Only the instructions `RUN`, `COPY`, `ADD` create layers. Other instructions
create temporary intermediate images, and do not increase the size of the build.
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
@ -308,7 +309,7 @@ alphanumerically. 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):
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 \
@ -323,65 +324,63 @@ RUN apt-get update && apt-get install -y \
### 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
Dockerfile, executing each in the order specified. As each instruction is
examined, Docker looks for an existing image in its cache that it can reuse,
rather than creating a new (duplicate) image.
rather than creating a new, duplicate image.
If you do not want to use the cache at all, you can use the `--no-cache=true`
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 is important to understand when it can, and cannot, find a matching
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 is already in the cache, the next
- 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
- 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 the file(s)
- 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 the file(s) are not considered in
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 the file(s), such
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 does not look at the
- 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
are not examined to determine if a cache hit exists. In that case just
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 is not used.
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`.
maintainable Dockerfile.
### FROM
[Dockerfile reference for the FROM instruction](../../engine/reference/builder.md#from)
Whenever possible, use current official images as the basis for your
images. We recommend the [Alpine image](https://hub.docker.com/_/alpine/) as it
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.
### LABEL
For more information about the `FROM` instruction, see [Dockerfile reference for the FROM instruction](../../engine/reference/builder.md#from).
[Understanding object labels](../../config/labels-custom-metadata.md)
### 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` and 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.
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
@ -395,14 +394,14 @@ 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.
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 can also be written as:
The above example can also be written as:
```dockerfile
# Set multiple labels at once, using line-continuation characters to break long lines
@ -421,16 +420,16 @@ See also [LABEL](../../engine/reference/builder.md#label) in the Dockerfile refe
### RUN
[Dockerfile reference for the RUN instruction](../../engine/reference/builder.md#run)
Split long or complex `RUN` statements on multiple lines separated with
backslashes to make your `Dockerfile` more readable, understandable, and
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 gotchas to
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`
@ -445,8 +444,7 @@ RUN apt-get update && apt-get install -y \
```
Using `apt-get update` alone in a `RUN` statement causes caching issues and
subsequent `apt-get install` instructions fail. For example, say you have a
Dockerfile:
subsequent `apt-get install` instructions fail. For example, the issue will occur in the following Dockerfile:
```dockerfile
# syntax=docker/dockerfile:1
@ -456,7 +454,7 @@ 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 extra package:
modify `apt-get install` by adding an extra package as shown in the following Dockerfile:
```dockerfile
# syntax=docker/dockerfile:1
@ -466,16 +464,16 @@ 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` is _not_ executed
because the build uses the cached version. Because the `apt-get update` is not
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:
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 \
@ -514,12 +512,11 @@ 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 is not stored in a layer. Since the
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.
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
@ -531,7 +528,7 @@ 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
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,
@ -542,6 +539,8 @@ build from inadvertently succeeding. For example:
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
@ -554,8 +553,6 @@ RUN set -o pipefail && wget -O - https://some.site | wc -l > /number
### CMD
[Dockerfile reference for the CMD instruction](../../engine/reference/builder.md#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
@ -572,9 +569,9 @@ conjunction with [`ENTRYPOINT`](../../engine/reference/builder.md#entrypoint), u
you and your expected users are already quite familiar with how `ENTRYPOINT`
works.
### EXPOSE
For more information about `CMD`, see [Dockerfile reference for the CMD instruction](../../engine/reference/builder.md#cmd).
[Dockerfile reference for the EXPOSE instruction](../../engine/reference/builder.md#expose)
### EXPOSE
The `EXPOSE` instruction indicates the ports on which a container listens
for connections. Consequently, you should use the common, traditional port for
@ -587,9 +584,9 @@ 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`).
### ENV
For more information about `EXPOSE`, see [Dockerfile reference for the EXPOSE instruction](../../engine/reference/builder.md#expose).
[Dockerfile reference for the ENV instruction](../../engine/reference/builder.md#env)
### 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
@ -610,9 +607,9 @@ RUN curl -SL https://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src
ENV PATH=/usr/local/postgres-$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
auto-magically bump the version of the software in your container.
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
@ -655,12 +652,10 @@ $ 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
- [Dockerfile reference for the ADD instruction](../../engine/reference/builder.md#add)
- [Dockerfile reference for the COPY instruction](../../engine/reference/builder.md#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
@ -668,9 +663,9 @@ 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
If you have multiple Dockerfile steps that use different files from your
context, `COPY` them individually, rather than all at once. This ensures that
each step's build cache is only invalidated (forcing the step to be re-run) if
each step's build cache is only invalidated, forcing the step to be re-run if
the specifically required files change.
For example:
@ -705,31 +700,34 @@ RUN mkdir -p /usr/src/things \
&& make -C /usr/src/things all
```
For other items (files, directories) that do not require `ADD`’s tar
auto-extraction capability, you should always use `COPY`.
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
[Dockerfile reference for the ENTRYPOINT instruction](../../engine/reference/builder.md#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).
image to be run as though it was that command, and then use `CMD` as the
default flags.
Let's start with an example of an image for the command line tool `s3cmd`:
The following is an example of an image for the command line tool `s3cmd`:
```dockerfile
ENTRYPOINT ["s3cmd"]
CMD ["--help"]
```
Now the image can be run like this to show the command's help:
You can use the following command to run the image and show the command's help:
```console
$ docker run s3cmd
```
Or using the right parameters to execute a command:
Or, you can use the right parameters to execute a command, like in the following example:
```console
$ docker run s3cmd ls s3://mybucket
@ -762,14 +760,10 @@ fi
exec "$@"
```
> Configure app as PID 1
>
> 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, see the [`ENTRYPOINT` reference](../../engine/reference/builder.md#entrypoint).
The helper script is copied into the container and run via `ENTRYPOINT` on
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, helper script is copied into the container and run via `ENTRYPOINT` on
container start:
```dockerfile
@ -798,33 +792,37 @@ Lastly, it could also be used to start a totally different tool, such as Bash:
$ docker run --rm -it postgres bash
```
For more information about `ENTRYPOINT`, see [Dockerfile reference for the ENTRYPOINT instruction](../../engine/reference/builder.md#entrypoint).
### VOLUME
[Dockerfile reference for the VOLUME instruction](../../engine/reference/builder.md#volume)
The `VOLUME` instruction should be used to expose any database storage area,
configuration storage, or files/folders created by your docker container. You
are strongly encouraged to use `VOLUME` for any mutable and/or user-serviceable
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
[Dockerfile reference for the USER instruction](../../engine/reference/builder.md#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:
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
```
> Consider an explicit UID/GID
> **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
@ -840,43 +838,45 @@ running it as non-`root`, consider using [“gosu”](https://github.com/tianon/
Lastly, to reduce layers and complexity, avoid switching `USER` back and forth
frequently.
### WORKDIR
For more information about `USER`, see [Dockerfile reference for the USER instruction](../../engine/reference/builder.md#user).
[Dockerfile reference for the WORKDIR instruction](../../engine/reference/builder.md#workdir)
### 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
[Dockerfile reference for the ONBUILD instruction](../../engine/reference/builder.md#onbuild)
An `ONBUILD` command executes after the current `Dockerfile` build completes.
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 the parent `Dockerfile` gives
to the child `Dockerfile`.
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`.
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).
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:
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
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.
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 `Dockerfile`s:
These Official Images have exemplary Dockerfiles:
* [Go](https://hub.docker.com/_/golang/)
* [Perl](https://hub.docker.com/_/perl/)