ci: Tag assets image with assets shasum

What does this MR do and why?

This does two things:

• Tag the assets image with the assets hash sum computed from the previous compile-production-assets job
• Skip the image building if the image already exists

Image exists?	Job	Duration
No	https://gitlab.com/gitlab-org/gitlab/-/jobs/3164087549	1m 39s
Yes	https://gitlab.com/gitlab-org/gitlab/-/jobs/3164171626	50s (50% improvement)

TODO:

• Support e2e:package-and-test where omnibus-gitlab assumes the assets version is GITLAB_REF_SLUG, which is set to CI_COMMIT_REF_SLUG in trigger-build.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #208389 (closed)