Enable dependabot automation for registry container image

[austinvazquez]

Use case

My use case runs registry in CI to test SOCI snapshotter (a containerd lazy loading snapshotter) pulling images from an OCI container registry.

Issue

We have dependabot automation configured to update the registry container image but it requires org.opencontainers.image.source label on the container image and tagging the repository with the same tags as the published container images.

Tasks

[ ] Add label to container image
[ ] Add release tag to match container image tags.

Additional context

1. https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file?learn=dependency_version_updates&learnProduct=code-security#docker
2. https://github.com/dependabot-fixtures/docker-with-source

[austinvazquez]

cc @milosgajdos @thaJeztah