Add container_digest as output variable.

[JeroenKnoops]

Proposal

In order to sign images, we need the container digest.

See sample code: https://github.com/philips-labs/slsa-provenance-action/blob/main/.github/workflows/ci.yaml#L113

Copy of sample code:

          export CONTAINER_DIGEST=$(make container-digest GITHUB_REF=${{ github.ref_name }})
          echo "::set-output name=container_digest::$CONTAINER_DIGEST"
          echo "::set-output name=container_tags::$(make container-tags CONTAINER_DIGEST="${CONTAINER_DIGEST}" | paste -s -d ',' -)"
          echo "::set-output name=container_repos::$(make container-repos CONTAINER_DIGEST="${CONTAINER_DIGEST}" | jq --raw-input . | jq --slurp -c)"

Suggested changes

Add outputs:

• container_digest
• container_tags
• container_repos

Related issue

Part of the first approach in #78 .

[JeroenKnoops]

We don't need the container_repo

[JeroenKnoops]

We also need an output flag which indicates whether the action has pushed an image to a docker registry.

Depending on that flag, the provenance / signing action can be triggered.

I will create a new issue for this.