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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 | ||
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| # Best Practices for Writing `Dockerfile`s | ||
|
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| ## Overview | ||
|
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| 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. `Dockerfile`s adhere to a specific format and use a | ||
| specific set of instructions. You can learn the basics on the | ||
| [Dockerfile Reference](https://docs.docker.com/reference/builder/) page. If | ||
| you’re new to writing `Dockerfile`s, you should start there. | ||
|
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| This document covers the best practices and methods recommended by Docker, | ||
| Inc. and the Docker Community for creating easy-to-use, effective | ||
| `Dockerfile`s. We strongly suggest you follow these recommendations (in fact, | ||
| if you’re creating an Official Image, you *must* adhere to these practices). | ||
|
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| You can see many of these practices and recommendations in action in the [buildpack-deps `Dockerfile`](https://github.com/docker-library/buildpack-deps/blob/master/jessie/Dockerfile). | ||
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| > Note: for more detailed explanations of any of the Dockerfile commands | ||
| >mentioned here, visit the [Dockerfile Reference](https://docs.docker.com/reference/builder/) page. | ||
| ## General Guidelines and Recommendations | ||
|
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| ### Containers should be ephemeral | ||
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| 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](https://docs.docker.com/reference/builder/#the-dockerignore-file) | ||
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| 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 | ||
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| 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 | ||
|
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| 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](https://docs.docker.com/userguide/dockerlinks/). | ||
|
|
||
| ### Minimize the number of layers | ||
|
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| 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 | ||
|
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| 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. | ||
|
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| Here’s an example from the [`buildpack-deps` image](https://github.com/docker-library/buildpack-deps): | ||
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| RUN apt-get update && apt-get install -y \ | ||
| bzr \ | ||
| cvs \ | ||
| git \ | ||
| mercurial \ | ||
| subversion | ||
|
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| ## The `Dockerfile` instructions | ||
|
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| This section contains specific recommendations for the correct usage of the | ||
| various instructions contained in a `Dockerfile`. | ||
|
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||
| ### [`FROM`](https://docs.docker.com/reference/builder/#from) | ||
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| Whenever possible, use Official Repositories as the basis for your image. We | ||
| recommend the [Debian image](https://registry.hub.docker.com/_/debian/) since | ||
| it’s very tightly controlled and kept extremely minimal (currently under 100 | ||
| mb), while still being a full distribution. | ||
|
|
||
| ### [`RUN`](https://docs.docker.com/reference/builder/#run) | ||
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| As always, to make your `Dockerfile` more readable, understandable, and | ||
| maintainable, put long or complex `RUN` statements on multiple lines separated with | ||
| backslashes. | ||
|
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| 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: | ||
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| * Don’t do `RUN apt-get update` on a single line. This will cause | ||
| caching issues if the referenced archive gets updated, which will make your | ||
| subsequent `apt-get install` fail without comment. | ||
|
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| * 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`. | ||
|
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| * Avoid `RUN apt-get upgrade` or `dist-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, use `apt-get install -y foo` and it will update automatically. | ||
|
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| * 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 including `apt-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. | ||
|
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| * 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 your `apt-get` code this way will greatly ease maintenance and reduce | ||
| failures due to unanticipated changes in required packages. | ||
|
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| #### Example | ||
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| 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). | ||
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| 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* | ||
|
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| ### [`CMD`](https://docs.docker.com/reference/builder/#cmd) | ||
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| 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. | ||
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| 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`](https://docs.docker.com/reference/builder/#entrypoint), unless | ||
| you and your expected users are already quite familiar with how `ENTRYPOINT` | ||
| works. | ||
|
|
||
| ### [`EXPOSE`](https://docs.docker.com/reference/builder/#expose) | ||
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| 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. | ||
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| 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`). | ||
|
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| ### [`ENV`](https://docs.docker.com/reference/builder/#env) | ||
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| 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:$PATH` will ensure that `CMD [“nginx”]` | ||
| just works. | ||
|
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| The `ENV` instruction is also useful for providing required environment | ||
| variables specific to services you wish to containerize, such as Postgres’s | ||
| `PGDATA`. | ||
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| Lastly, `ENV` can also be used to set commonly used version numbers so that | ||
| version bumps are easier to maintain, as seen in the following example: | ||
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| ENV PG_MAJOR 9.3 | ||
| ENV PG_VERSION 9.3.4 | ||
| RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && … | ||
| ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH | ||
|
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| 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`](https://docs.docker.com/reference/builder/#add) or [`COPY`](https://docs.docker.com/reference/builder/#copy) | ||
|
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| 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 /`. | ||
|
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| 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: | ||
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| 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 | ||
|
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| And instead, do something like: | ||
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| 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 | ||
|
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| For other items (files, directories) that do not require `ADD`’s tar | ||
| auto-extraction capability, you should always use `COPY`. | ||
|
|
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| ### [`ENTRYPOINT`](https://docs.docker.com/reference/builder/#entrypoint) | ||
|
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| 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. | ||
|
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| 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`). | ||
|
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| For example, let’s look at the `Dockerfile` for the | ||
| [Postgres Official Image](https://github.com/docker-library/postgres). | ||
| It refers to the following script: | ||
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| ```bash | ||
| #!/bin/bash | ||
| set -e | ||
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| if [ "$1" = 'postgres' ]; then | ||
| chown -R postgres "$PGDATA" | ||
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| if [ -z "$(ls -A "$PGDATA")" ]; then | ||
| gosu postgres initdb | ||
| fi | ||
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| exec gosu postgres "$@" | ||
| fi | ||
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| exec "$@" | ||
| ``` | ||
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| That script then gets copied into the container and the run via `ENTRYPOINT` on | ||
| container startup: | ||
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| COPY ./docker-entrypoint.sh / | ||
| ENTRYPOINT ["/docker-entrypoint.sh"] | ||
|
|
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| ### [`VOLUME`](https://docs.docker.com/reference/builder/#volume) | ||
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| 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. | ||
|
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| ### [`USER`](https://docs.docker.com/reference/builder/#user) | ||
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| 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`. | ||
|
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| >**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”](https://github.com/tianon/gosu). | ||
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| Lastly, to reduce layers and complexity, try to minimize switching `USER` back | ||
| and forth frequently. | ||
|
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| ### [`WORKDIR`](https://docs.docker.com/reference/builder/#workdir) | ||
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| 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. | ||
|
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| ### [`ONBUILD`](https://docs.docker.com/reference/builder/#onbuild) | ||
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| `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](https://github.com/docker-library/ruby/blob/master/2.1/onbuild/Dockerfile). | ||
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| Images built from `ONBUILD` should get a separate tag, for example: | ||
| `ruby:1.9-onbuild` or `ruby:2.0-onbuild`. | ||
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| 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. | ||
|
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| ## Examples For Official Repositories | ||
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| These Official Repos have exemplary `Dockerfile`s: | ||
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| * [Go](https://registry.hub.docker.com/_/golang/) | ||
| * [Perl](https://registry.hub.docker.com/_/perl/) | ||
| * [Hy](https://registry.hub.docker.com/_/hylang/) | ||
| * [Rails](https://registry.hub.docker.com/_/rails) | ||
|
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| ## Additional Resources: | ||
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| * [Dockerfile Reference](https://docs.docker.com/reference/builder/#onbuild) | ||
| * [More about Base Images](https://docs.docker.com/articles/baseimages/) | ||
| * [More about Automated Builds](https://docs.docker.com/docker-hub/builds/) | ||
| * [Guidelines for Creating Official Repositories](https://docs.docker.com/docker-hub/official_repos/) |
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