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page_title: Best Practices for Writing Dockerfiles page_description: Hints, tips and guidelines for writing clean, reliable Dockerfiles page_keywords: Examples, Usage, base image, docker, documentation, dockerfile, best practices, hub, official repo
Best Practices for Writing Dockerfiles
Overview
Docker can build images automatically by reading the instructions from a
Dockerfile, a text file that contains all the commands, in order, needed to
build a given image. Dockerfiles adhere to a specific format and use a
specific set of instructions. You can learn the basics on the
Dockerfile Reference page. If
you’re new to writing Dockerfiles, you should start there.
This document covers the best practices and methods recommended by Docker,
Inc. and the Docker Community for creating easy-to-use, effective
Dockerfiles. We strongly suggest you follow these recommendations (in fact,
if you’re creating an Official Image, you must adhere to these practices).
You can see many of these practices and recommendations in action in the buildpack-deps Dockerfile.
Note: for more detailed explanations of any of the Dockerfile commands mentioned here, visit the Dockerfile Reference page.
General Guidelines and Recommendations
Containers should be ephemeral
The container produced by the image your Dockerfile defines should be as
ephemeral as possible. “Ephemeral” here means that it can be stopped and
destroyed and a new one built and put in place with an absolute minimum of
set-up and configuration.
Use a .dockerignore file
For faster uploading and efficiency during docker build, you should make use
of a .dockerignore file to exclude files or directories from the build
context and final image. For example, unless.git is needed by your build
process or scripts, you should add it to .dockerignore, which can save many
megabytes worth of upload time.
Avoid installing unnecessary packages
You should 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.
Run only one process per container
In almost all cases, you should only run a single process in a single container. Decoupling applications into multiple containers makes it much easier to scale horizontally and reuse containers. If that service depends on another service, make use of container linking.
Minimize the number of layers
You need to find the balance between readability (and thus long-term
maintainability) of the Dockerfile and minimizing the number of layers it
uses. Be strategic and cautious about the number of layers you use.
Sort multi-line arguments
Whenever possible, ease later changes by sorting multi-line arguments
alphanumerically. This will help you 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:
RUN apt-get update && apt-get install -y \
bzr \
cvs \
git \
mercurial \
subversion
The Dockerfile instructions
This section contains specific recommendations for the correct usage of the
various instructions contained in a Dockerfile.
FROM
Whenever possible, use Official Repositories as the basis for your image. We recommend the Debian image since it’s very tightly controlled and kept extremely minimal (currently under 100 mb), while still being a full distribution.
RUN
As always, to make your Dockerfile more readable, understandable, and
maintainable, put long or complex RUN statements on multiple lines separated with
backslashes.
Probably the most common use-case for RUN is an application of apt-get.
When using apt-get, here a few things to keep in mind:
-
Don’t do
RUN apt-get updateon a single line. This will cause caching issues if the referenced archive gets updated, which will make your subsequentapt-get installfail without comment. -
For the most part, to keep your code more readable and maintainable, avoid
RUN apt-get install -y package-foo && apt-get install -y package-bar. -
Avoid
RUN apt-get upgradeordist-upgrade, since many of the “essential” packages from the base images will fail to upgrade inside an unprivileged container. If a base package is out of date, you should contact its maintainers. If you know there’s a particular package,foo, that needs to be updated, useapt-get install -y fooand it will update automatically. -
Do use
RUN apt-get update && apt-get install -y package-bar package-foo package-baz. Writing the instruction this way not only makes it easier to read and maintain, but also, by includingapt-get update, ensures that the cache will naturally be busted and the latest versions will be installed with no further coding or manual intervention required. -
Further natural cache-busting can be realized by version-pinning packages (e.g.,
package-foo=1.3.*). This will force retrieval of that version regardless of what’s in the cache. Forming yourapt-getcode this way will greatly ease maintenance and reduce failures due to unanticipated changes in required packages.
Example
Below is a well-formed RUN instruction that demonstrates the above
recommendations. Note that the last package, s3cmd, specifies a version
1.1.0*. If the image previously used an older version, specifying the new one
will cause a cache bust of apt-get update and ensure the installation of
the new version (which in this case had a new, required feature).
RUN apt-get update && apt-get install -y \
aufs-tools \
automake \
btrfs-tools \
build-essential \
curl \
dpkg-sig \
git \
iptables \
libapparmor-dev \
libcap-dev \
libsqlite3-dev \
lxc=1.0* \
mercurial \
parallel \
reprepro \
ruby1.9.1 \
ruby1.9.1-dev \
s3cmd=1.1.0*
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 (Apache, Rails, etc.), 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 (bash, python,
perl, etc), 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, unless
you and your expected users are already quite familiar with how ENTRYPOINT
works.
EXPOSE
The EXPOSE instruction indicates the ports on which a container will listen
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
In order 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:$PATH will ensure that CMD [“nginx”]
just works.
The ENV instruction is also useful for providing required environment
variables specific to services you wish to containerize, such as Postgres’s
PGDATA.
Lastly, ENV can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example:
ENV PG_MAJOR 9.3
ENV PG_VERSION 9.3.4
RUN curl -SL http://example.com/postgres-$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.
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 /.
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 won'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 mdkir -p /usr/src/things \
&& curl -SL http://example.com/big.tar.gz \
| 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
The best use for ENTRYPOINT is as a helper script. Using ENTRYPOINT for
other tasks can make your code harder to understand. For example,
docker run -it official-image bash is much easier to understand than
docker run -it --entrypoint bash official-image -i, especially for Docker
beginners.
In order to avoid a situation where commands are run without clear visibility
to the user, make sure your script ends with something like exec "$@". After
the entrypoint completes, the script will transparently bootstrap the command
invoked by the user, making what has been run clear to the user (for example,
docker run -it mysql mysqld --some --flags will transparently run
mysqld --some --flags after ENTRYPOINT runs initdb).
For example, let’s look at the Dockerfile for the
Postgres Official Image.
It refers to the following script:
#!/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 "$@"
fi
exec "$@"
That script then gets copied into the container and the run via ENTRYPOINT on
container startup:
COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
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
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 -r -g postgres postgres.
Note
that users/groups in an image get assigned a non-deterministic UID/GID in that the “next” UID/GID gets assigned regardless of image rebuilds. So, if it’s critical, you should assign an explicit UID/GID.
You should avoid installing or using sudo since it has unpredictable TTY and
signal-forwarding behavior that can cause more more problems than it solves. If
you absolutely need functionality similar to sudo (e.g., initializing the
daemon as root but running it as non-root), you may be able to use
“gosu”.
Lastly, to reduce layers and complexity, try to minimize switching USER back
and forth frequently.
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
ONBUILD is only 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.
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 will
fail catastrophically if the new build's context is missing the resource being
added. Adding a separate tag, as recommended above, will help mitigate this by
allowing the Dockerfile author to make a choice.
Examples For Official Repositories
These Official Repos have exemplary Dockerfiles: