Docker Images

TaskCluster Docker images are defined in the source directory under taskcluster/docker. Each directory therein contains the name of an image used as part of the task graph.


Each folder describes a single docker image. We have two types of images that can be defined:

  1. Task Images (build-on-push)

  2. Docker Images (prebuilt)

These images depend on one another, as described in the FROM line at the top of the Dockerfile in each folder.

Images could either be an image intended for pushing to a docker registry, or one that is meant either for local testing or being built as an artifact when pushed to vcs.

Task Images (build-on-push)

Images can be uploaded as a task artifact, indexed under a given namespace, and used in other tasks by referencing the task ID.

Important to note, these images do not require building and pushing to a docker registry, and are built per push (if necessary) and uploaded as task artifacts.

The decision task that is run per push will determine if the image needs to be built based on the hash of the context directory and if the image exists under the namespace for a given branch.

As an additional convenience, and a precaution to loading images per branch, if an image has been indexed with a given context hash for mozilla-central, any tasks requiring that image will use that indexed task. This is to ensure there are not multiple images built/used that were built from the same context. In summary, if the image has been built for mozilla-central, pushes to any branch will use that already built image.

To use within an in-tree task definition, the format is:

    type: 'task-image'
    path: 'public/image.tar.zst'
    taskId: '<task_id_for_image_builder>'

Context Directory Hashing

Decision tasks will calculate the sha256 hash of the contents of the image directory and will determine if the image already exists for a given branch and hash or if a new image must be built and indexed.

Note: this is the contents of only the context directory, not the image contents.

The decision task will:

  1. Recursively collect the paths of all files within the context directory

  2. Sort the filenames alphabetically to ensure the hash is consistently calculated

  3. Generate a sha256 hash of the contents of each file

  4. All file hashes will then be combined with their path and used to update the hash of the context directory

This ensures that the hash is consistently calculated and path changes will result in different hashes being generated.

Task Image Index Namespace

Images that are built on push and uploaded as an artifact of a task will be indexed under the following namespaces.

  • gecko.cache.level-{level}.docker.v2.{name}.hash.{digest}

  • gecko.cache.level-{level}.docker.v2.{name}.latest

  • gecko.cache.level-{level}.docker.v2.{name}.pushdate.{year}.{month}-{day}-{pushtime}

Not only can images be browsed by the pushdate and context hash, but the ‘latest’ namespace is meant to view the latest built image. This functions similarly to the ‘latest’ tag for docker images that are pushed to a registry.

Docker Registry Images (prebuilt)

*Warning: Registry images are only used for ``decision`` and ``image_builder`` images.*

These are images that are intended to be pushed to a docker registry and used by specifying the docker image name in task definitions. They are generally referred to by a <repo>@<repodigest> string:


image: taskcluster/decision:0.1.10@sha256:c5451ee6c655b3d97d4baa3b0e29a5115f23e0991d4f7f36d2a8f793076d6854

Such images must always be referred to with both a version and a repo digest. For the decision image, the repo digest is stored in the HASH file in the image directory and used to refer to the image as above. The version for both images is in VERSION.

The version file serves to help users identify which image is being used, and makes old versions easy to discover in the registry.

The file taskcluster/docker/REGISTRY specifies the image registry to which the completed image should be uploaded.

Docker Hashes and Digests

There are several hashes involved in this process:

  • Image Hash – the long version of the image ID; can be seen with docker images --no-trunc or in the Id field in docker inspect.

  • Repo Digest – hash of the image manifest; seen when running docker push or docker pull.

  • Context Directory Hash – see above (not a Docker concept at all)

The use of hashes allows older tasks which were designed to run on an older version of the image to be executed in Taskcluster while new tasks use the new version. Furthermore, this mitigates attacks against the registry as docker will verify the image hash when loading the image.

(Re)-Building images

Generally, images can be pulled from the Docker registry rather than built locally, however, for developing new images it’s often helpful to hack on them locally.

To build an image, invoke mach taskcluster-build-image with the name of the folder (without a trailing slash):

./mach taskcluster-build-image <image-name>

This is a wrapper around docker build -t $REGISTRY/$FOLDER:$VERSION.

It’s a good idea to bump the VERSION early in this process, to avoid docker push-ing over any old tags.

For task images, test your image locally or push to try. This is all that is required.

Docker Registry Images

Landing docker registry images takes a little more care.

Begin by bumping the VERSION. Once the new version of the image has been built and tested locally, push it to the docker registry and make note of the resulting repo digest. Put this value in the HASH file for the decision image and in taskcluster/gecko_taskgraph/transforms/ for the image_builder image.

The change is now safe to use in Try pushes.

Note that image_builder change can be tested directly in try pushes without using a registry, as the in-registry image_builder image is used to build a task image which is then used to build other images. It is referenced by hash in taskcluster/gecko_taskgraph/transforms/

Special Dockerfile Syntax

Dockerfile syntax has been extended to allow any file from the source checkout to be added to the image build context. (Traditionally you can only ADD files from the same directory as the Dockerfile.)

Simply add the following syntax as a comment in a Dockerfile:

# %include <path>


# %include mach # %include testing/mozharness

The argument to # %include is a relative path from the root level of the source directory. It can be a file or a directory. If a file, only that file will be added. If a directory, every file under that directory will be added (even files that are untracked or ignored by version control).

Files added using # %include syntax are available inside the build context under the topsrcdir/ path.

Files are added as they exist on disk. e.g. executable flags should be preserved. However, the file owner/group is changed to root and the mtime of the file is normalized.

Here is an example Dockerfile snippet:

# %include mach
ADD topsrcdir/mach /builds/worker/mach