Publish full image ref as result

[michaelsauter]

Right now only the digest is published. It would be nice to publish the image ref (including registry, image stream, etc.) to make use of it in other tasks. For example, one could have a separate task which signs the image (although signing could/should likely also be part of this task):

- name: sign
  runAfter:
    - package
  taskSpec:
    params:
      - { name: image-ref, type: 'string' }
    steps:
      - name: sign
        image: cgr.dev/chainguard/cosign
        imagePullPolicy: IfNotPresent
        script: |
          cosign sign --tlog-upload=false --key k8s://<namespace>/<secret> $(params.image-ref)
  params:
    - { name: image-ref, value: '$(tasks.package.results.image-ref)' }