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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. By “ephemeral,” we mean 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
In most cases, it's best to put each Dockerfile in an empty directory. Then,
add to that directory only the files needed for building the Dockerfile. To
increase the build's performance, you can exclude files and directories by
adding a .dockerignore file to that directory as well. This file supports
exclusion patterns similar to .gitignore files. For information on creating one,
see the .dockerignore file.
Avoid installing unnecessary packages
In order to reduce complexity, dependencies, file sizes, and build times, 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
Build cache
During the process of building an image Docker will step through the
instructions in your Dockerfile executing each in the order specified.
As each instruction is examined Docker will look 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 then it is very important to understand when it will, and will not, find a matching image. The basic rules that Docker will follow are outlined below:
-
Starting with a base 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
Dockerfilewith one of the child images is sufficient. However, certain instructions require a little more examination and explanation. -
For the
ADDandCOPYinstructions, 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
ADDandCOPYcommands, cache checking will not look at the files in the container to determine a cache match. For example, when processing aRUN apt-get -y updatecommand the files updated in the container will not be examined to determine if a cache hit exists. In that case just the command string itself will be used to find a match.
Once the cache is invalidated, all subsequent Dockerfile commands will
generate new images and the cache will not be used.
The Dockerfile instructions
Below you'll find recommendations for the best way to write the
various instructions available for use in a Dockerfile.
FROM
Dockerfile reference for the FROM instruction
Whenever possible, use current 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
Dockerfile reference for the RUN instruction
As always, to make your Dockerfile more readable, understandable, and
maintainable, split long or complex RUN statements on multiple lines separated
with backslashes.
apt-get
Probably the most common use-case for RUN is an application of apt-get. The
RUN apt-get command, because it installs packages, has several gotchas to look
out for.
You should avoid RUN apt-get upgrade or dist-upgrade, as many of the
“essential” packages from the base images won't upgrade inside an unprivileged
container. If a package contained in the base image is out-of-date, you should
contact its maintainers.
If you know there’s 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 instructions specifies a version 1.1.0*. If the image previously
used an older version, specifying the new one causes a cache bust of apt-get update and ensure the installation of the new version. Listing packages on
each line can also prevent mistakes in package duplication.
In addition, cleaning up the apt cache and removing /var/lib/apt/lists helps
keep the image size down. Since the RUN statement starts with
apt-get update, the package cache will always be refreshed prior to
apt-get install.
Note
: The official Debian and Ubuntu images automatically run
apt-get clean, so explicit invocation is not required.
CMD
Dockerfile reference for the CMD instruction
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
Dockerfile reference for the EXPOSE instruction
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
Dockerfile reference for the ENV instruction
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
Dockerfile reference for the ADD instruction
Dockerfile reference for the COPY instruction
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 will ensure 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 /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 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 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
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
uses the following script as its ENTRYPOINT:
#!/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 "$@"
fi
exec "$@"
Note
: This script uses the
execBash command 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. See theENTRYPOINThelp for more details.
The helper script is copied into the container and run via ENTRYPOINT on
container start:
COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
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
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
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: Users and groups in an image get 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 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, avoid switching USER back
and forth frequently.
WORKDIR
Dockerfile reference for the WORKDIR instruction
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
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.
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 Repositories have exemplary Dockerfiles: