CI: improve image cache handling

What does this MR do?

CI: improve image cache handling

Update CACHE_IMAGE handling, adding some output and clarified logic to the process.

• Attempting to cache on the branch / tag
• If not able to pull cache from previous runs of the same pipeline,
  • if stable branch or a tag, disable
  • otherwise, attempt to use CI_DEFAULT_BRANCH

CI maintenance, no changelog entry

Related issues

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

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