Change the image tag to be the MD5 of assets files in `build-assets-image` and skip image building if the image exists

The following discussion from !26008 (merged) should be addressed:

• Albert started a discussion:

    - time bin/rake gitlab:assets:compile > assets-compile.log 2>&1
    - compile_webpack_assets_if_needed
    - bin/rake gitlab:assets:fix_urls
    # TODO: Change the image tag to be the MD5 of assets files and skip image building if the image exists
    # We'll also need to pass GITLAB_ASSETS_TAG to the trigerred omnibus-gitlab pipeline similarly to how we do it for trigerred CNG pipelines

The goal is to speed up the build-assets-image job by avoiding rebuilding the image if there's already an image with the exact same assets.