Automation with content trust

Estimated reading time: 2 minutes

Your automation systems that pull or build images can also work with trust. Any automation environment must set DOCKER_TRUST_ENABLED either manually or in a scripted fashion before processing images.

Bypass requests for passphrases

To allow tools to wrap docker and push trusted content, there are two environment variables that allow you to provide the passphrases without an expect script, or typing them in:

  • DOCKER_CONTENT_TRUST_ROOT_PASSPHRASE
  • DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE

Docker attempts to use the contents of these environment variables as passphrase for the keys. For example, an image publisher can export the repository target and snapshot passphrases:

$  export DOCKER_CONTENT_TRUST_ROOT_PASSPHRASE="u7pEQcGoebUHm6LHe6"
$  export DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE="l7pEQcTKJjUHm6Lpe4"

Then, when pushing a new tag the Docker client does not request these values but signs automatically:

$  docker push docker/trusttest:latest
The push refers to a repository [docker.io/docker/trusttest] (len: 1)
a9539b34a6ab: Image already exists
b3dbab3810fc: Image already exists
latest: digest: sha256:d149ab53f871 size: 3355
Signing and pushing trust metadata

When working directly with the Notary client, it will use its own set of environment variables.

Building with content trust

You can also build with content trust. Before running the docker build command, you should set the environment variable DOCKER_CONTENT_TRUST either manually or in a scripted fashion. Consider the simple Dockerfile below.

FROM docker/trusttest:latest
RUN echo

The FROM tag is pulling a signed image. You cannot build an image that has a FROM that is not either present locally or signed. Given that content trust data exists for the tag latest, the following build should succeed:

$  docker build -t docker/trusttest:testing .
Using default tag: latest
latest: Pulling from docker/trusttest

b3dbab3810fc: Pull complete
a9539b34a6ab: Pull complete
Digest: sha256:d149ab53f871

If content trust is enabled, building from a Dockerfile that relies on tag without trust data, causes the build command to fail:

$  docker build -t docker/trusttest:testing .
unable to process Dockerfile: No trust data for notrust
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