Update publish-release task to support release signing

[ghost]

Feature request

Sign the images that we publish during a release.

More Info

Over in the Plumbing repo we now have a documented guide to signing Tekton releases in the dogfooding cluster.

In order to support signing we need to update our image publishing machinery to emit a result with name IMAGES and a given format. I believe this is the task we'll need to update: https://github.com/tektoncd/pipeline/blob/main/tekton/publish.yaml

[priyawadhwa]

Awesome! Yup, IMAGES will work -- basically it expects a comma-separated list of images built in that TaskRun, qualified by digest.

The chains publish.yaml might be a helpful example.