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---
description: Hints, tips and guidelines for writing clean, reliable Dockerfiles
keywords: parent image, images, dockerfile, best practices, hub, official repo
redirect_from:
- /articles/dockerfile_best-practices/
- /engine/articles/dockerfile_best-practices/
- /docker-cloud/getting-started/intermediate/optimize-dockerfiles/
- /docker-cloud/tutorials/optimize-dockerfiles/
- /engine/userguide/eng-image/dockerfile_best-practices/
title: Best practices for writing Dockerfiles
---
This document 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/).
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`:
```conf
FROM ubuntu:15.04
COPY . /app
RUN make /app
CMD python /app/app.py
```
Each instruction creates one layer:
- `FROM` creates a layer from the `ubuntu:15.04` 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_
(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 thin writable container layer.
For more on image layers (and how Docker builds and stores images), see
[About storage drivers](/storage/storagedriver/).
## 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
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
When you issue a `docker build` command, the current working directory is called
the _build context_. By default, the Dockerfile is assumed to be located here,
but you can specify a different location with the file flag (`-f`). Regardless
of where the `Dockerfile` actually lives, all recursive contents of files and
directories in the current directory are sent to the Docker daemon as the build
context.
> Build context example
>
> Create a directory for the build context and `cd` into it. Write "hello" into
> a text file named `hello` and create a Dockerfile that runs `cat` on it. Build
> the image from within the build context (`.`):
>
> ```shell
> mkdir myproject && cd myproject
> echo "hello" > hello
> echo -e "FROM busybox\nCOPY /hello /\nRUN cat /hello" > Dockerfile
> docker build -t helloapp:v1 .
> ```
>
> Move `Dockerfile` and `hello` into separate directories and build a second
> version of the image (without relying on cache from the last build). Use `-f`
> to point to the Dockerfile and specify the directory of the build context:
>
> ```shell
> mkdir -p dockerfiles context
> mv Dockerfile dockerfiles && mv hello context
> docker build --no-cache -t helloapp:v2 -f dockerfiles/Dockerfile context
> ```
Inadvertently including files that are not necessary for building an image
results in a larger build context and larger image size. This can increase the
time to build the image, time to pull and push it, and the container runtime
size. To see how big your build context is, look for a message like this when
building your `Dockerfile`:
```none
Sending build context to Docker daemon 187.8MB
```
### Pipe Dockerfile through `stdin`
Docker 17.05 added the ability to build images by piping `Dockerfile` through
`stdin` with a _local or remote build-context_. In earlier versions, building an
image with a `Dockerfile` from `stdin` did not send the build-context.
**Docker 17.04 and lower**
```
docker build -t foo -<<EOF
FROM busybox
RUN echo "hello world"
EOF
```
**Docker 17.05 and higher (local build-context)**
```
docker build -t foo . -f-<<EOF
FROM busybox
RUN echo "hello world"
COPY . /my-copied-files
EOF
```
**Docker 17.05 and higher (remote build-context)**
```
docker build -t foo https://github.com/thajeztah/pgadmin4-docker.git -f-<<EOF
FROM busybox
COPY LICENSE config_local.py /usr/local/lib/python2.7/site-packages/pgadmin4/
EOF
```
### 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
[.dockerignore file](/engine/reference/builder.md#dockerignore-file).
### Use multi-stage builds
[Multi-stage builds](multistage-build.md) (in [Docker 17.05](/release-notes/docker-ce/#17050-ce-2017-05-04) or higher)
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, you can order them from the
less frequently changed (to ensure the build cache is reusable) to the more
frequently changed:
* Install tools you need to build your application
* Install or update library dependencies
* Generate your application
A Dockerfile for a Go application could look like:
```
FROM golang:1.9.2-alpine3.6 AS build
# Install tools required for project
# Run `docker build --no-cache .` to update dependencies
RUN apk add --no-cache git
RUN go get github.com/golang/dep/cmd/dep
# List project dependencies with Gopkg.toml and Gopkg.lock
# These layers are only re-built when Gopkg files are updated
COPY Gopkg.lock Gopkg.toml /go/src/project/
WORKDIR /go/src/project/
# Install library dependencies
RUN dep ensure -vendor-only
# 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
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.
### 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 is 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](http://www.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](/engine/userguide/networking/)
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:
- In Docker 1.10 and higher, only the instructions `RUN`, `COPY`, `ADD` create
layers. Other instructions create temporary intermediate images, and do not
directly increase the size of the build.
- In Docker 17.05 and higher, you can do [multi-stage builds](multistage-build.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, ease later changes by sorting multi-line arguments
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):
RUN apt-get update && apt-get install -y \
bzr \
cvs \
git \
mercurial \
subversion
### 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 that it can reuse,
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`
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
image. The basic rules that Docker follows are outlined below:
- Starting with a parent image that is 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 the file(s)
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
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
as the contents and metadata, then the cache is invalidated.
- Aside from the `ADD` and `COPY` commands, cache checking does not 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
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.
## Dockerfile instructions
These recommendations are designed to help you create an efficient and
maintainable `Dockerfile`.
### FROM
[Dockerfile reference for the FROM instruction](/engine/reference/builder.md#from)
Whenever possible, use current official repositories as the basis for your
images. We recommend the [Alpine image](https://hub.docker.com/_/alpine/) as it
is tightly controlled and small in size (currently under 5 MB), while still
being a full Linux distribution.
### LABEL
[Understanding object labels](/config/labels-custom-metadata.md)
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.
```conf
# 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.
```conf
# 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:
```conf
# 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#managing-labels-on-objects). See also
[LABEL](/engine/reference/builder/#label) in the Dockerfile reference.
### 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
maintainable.
#### 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
look out for.
Avoid `RUN apt-get upgrade` and `dist-upgrade`, as many of the “essential”
packages from the parent images cannot upgrade inside an
[unprivileged container](/engine/reference/run.md#security-configuration). If a package
contained in the parent image is out-of-date, contact its maintainers. If you
know there is a particular package, `foo`, that needs to be updated, use
`apt-get install -y foo` to update automatically.
Always combine `RUN apt-get update` with `apt-get install` in the same `RUN`
statement. For example:
RUN apt-get update && apt-get install -y \
package-bar \
package-baz \
package-foo
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:
FROM ubuntu:14.04
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 extra package:
FROM ubuntu:14.04
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` is _not_ executed
because the build uses the cached version. Because the `apt-get update` is not
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:
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.
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 is not 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
```
> Not all shells support the `-o pipefail` option.
>
> In such cases (such as the `dash` shell, which is the default 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
[Dockerfile reference for the CMD instruction](/engine/reference/builder.md#cmd)
The `CMD` instruction should be used to run the software contained by 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.
### EXPOSE
[Dockerfile reference for the EXPOSE instruction](/engine/reference/builder.md#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`).
### ENV
[Dockerfile reference for the ENV instruction](/engine/reference/builder.md#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:$PATH` ensures that `CMD [“nginx”]`
just works.
The `ENV` instruction is also useful for providing required environment
variables specific to services you wish to containerize, such as Postgres’s
`PGDATA`.
Lastly, `ENV` can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example:
ENV PG_MAJOR 9.3
ENV PG_VERSION 9.3.4
RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
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.
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
FROM alpine
ENV ADMIN_USER="mark"
RUN echo $ADMIN_USER > ./mark
RUN unset ADMIN_USER
CMD sh
```
```bash
$ docker run --rm -it test sh 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
FROM alpine
RUN export ADMIN_USER="mark" \
&& echo $ADMIN_USER > ./mark \
&& unset ADMIN_USER
CMD sh
```
```bash
$ docker run --rm -it test sh echo $ADMIN_USER
```
### 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
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. This ensures that
each step's build cache is only invalidated (forcing the step to be re-run) if
the specifically required files change.
For example:
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:
ADD http://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:
RUN mkdir -p /usr/src/things \
&& curl -SL http://example.com/big.tar.xz \
| tar -xJC /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`.
### 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).
Let's start with an example of an image for the command line tool `s3cmd`:
ENTRYPOINT ["s3cmd"]
CMD ["--help"]
Now the image can be run like this to show the command's help:
$ docker run s3cmd
Or using the right parameters to execute a command:
$ 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 "$@"
fi
exec "$@"
```
> Configure app as PID 1
>
> This script uses [the `exec` Bash command](http://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
container start:
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:
$ docker run postgres
Or, it can be used to run Postgres and pass parameters to the server:
$ docker run postgres postgres --help
Lastly, it could also be used to start a totally different tool, such as Bash:
$ docker run --rm -it postgres bash
### 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
parts of your image.
### 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 `RUN groupadd -r postgres && useradd --no-log-init -r -g postgres postgres`.
> 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.
> 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.
### WORKDIR
[Dockerfile reference for the WORKDIR instruction](/engine/reference/builder.md#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.
### ONBUILD
[Dockerfile reference for the ONBUILD instruction](/engine/reference/builder.md#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 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/master/2.4/jessie/onbuild/Dockerfile).
Images built from `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.
## Examples for Official Repositories
These Official Repositories have exemplary `Dockerfile`s:
* [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 Base Images](baseimages.md)
* [More about Automated Builds](/docker-hub/builds/)
* [Guidelines for Creating Official Repositories](/docker-hub/official_repos/)
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