Pin image digests for more reliable review apps (version 2)

What does this MR do?

Version 2 of !3406 (merged)

Adds a job before the review-related jobs trigger. It calculates the correct image to use (using existing logic), and then uses skopeo to retrieve the current image digest. It then includes this digest in a Helm values file that is saved as an artifact so it's available to the review app jobs.

This should make the review apps more reliable, as this avoids the scenario in which the global.image.pullPolicy=Always could lead to review apps running containers that are misaligned

Related issues

Closes #4999 (closed)

Author 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, follow the instructions in the "Reviewer Roulette" section of the Danger Bot MR comment, as per the Distribution experimental MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added/updated
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Equivalent MR/issue for Gitlab Operator project opened (see Operator documentation on impact of Charts changes)
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.