[FEATURE] Provide image digest

[ctron]

Is your feature request related to a problem? Please describe.

I am trying to sign images with cosign. cosign wants a digest, instead of a tag (it can work with a tag today, but that is discouraged and deprecated).

Describe the solution you'd like

I would like the buildah step to set an output variable with the image digest.

Describe alternatives you've considered

I can manually try to evaluate the image/manifest digest.

Additional context

[np22-jpg]

I might be a little bit late, but redhat-actions/push-to-registry might provide what you're looking for.

Here's an example of signing images with cosign using push-to-registry from the ublue project.