Automation with content trust

Estimated reading time: 2 minutes

It is very common for Docker Content Trust to be built into existing automation systems. To allow tools to wrap Docker and push trusted content, there are environment variables that can be passed through to the client.

This guide follows the steps as described here so please read that and understand its prerequisites.

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

Adding a Delegation Private Key

To automate importing a delegation private key to the local Docker trust store, we need to pass a passphrase for the new key. This passphrase will be required everytime that delegation signs a tag.

$ export DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE="mypassphrase123"

$ docker trust key load delegation.key --name jeff
Loading key from "delegation.key"...
Successfully imported key from delegation.key

Adding a Delegation Public Key

If you initialising a repository at the same time as adding a Delegation public key, then you will need to use the local Notary Canonical Root Key’s passphrase to create the repositories trust data. If the repository has already been initiated then you only need the repositories passphrase.

# Export the Local Root Key Passphrase if required.
$ export DOCKER_CONTENT_TRUST_ROOT_PASSPHRASE="rootpassphrase123"

# Export the Repository Passphrase
$ export DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE="repopassphrase123"

# Initialise Repo and Push Delegation
$ docker trust signer add --key delegation.crt jeff dtr.example.com/admin/demo
Adding signer "jeff" to dtr.example.com/admin/demo...
Initializing signed repository for dtr.example.com/admin/demo...
Successfully initialized "dtr.example.com/admin/demo"
Successfully added signer: dtr.example.com/admin/demo

Signing an Image

Finally when signing an image, we will need to export the passphrase of the signing key. This was created when the key was loaded into the local Docker trust store with $ docker trust key load.

$ export DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE="mypassphrase123"

$ docker trust sign dtr.example.com/admin/demo:1
Signing and pushing trust data for local image dtr.example.com/admin/demo:1, may overwrite remote trust data
The push refers to repository [dtr.example.com/admin/demo]
428c97da766c: Layer already exists
2: digest: sha256:1a6fd470b9ce10849be79e99529a88371dff60c60aab424c077007f6979b4812 size: 524
Signing and pushing trust metadata
Successfully signed dtr.example.com/admin/demo:1

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
trust, security, docker, documentation, automation