Make cosign verification work in all scenarios

What does this MR do?

• Make cosign verification work with all branch names. Using CI_COMMIT_REF_SLUG meant that branch names like deps/e8b98d2-75e5507 got converted to deps-e8b98d2-75e5507 in the cosign verify command. This will fail as the signing used the branch name as-is - 1f85e2a5
• Ensure verification works even if image was not built in a job but reused from previous one - 75d84777
• Trigger signing and verification of images only after they have been pushed to the registry - 0803ccb3

Related issues

Closes #631 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io