Follow-up from "Fix wrong tag in `Test::Instance::Image`"

The following discussion from !72 (merged) should be addressed:

• @grzesiek started a discussion:

  That was too easy to miss. The problem stems from the fact that a tag accessor is no longer a single source of truth. We also have a release. This is a good bug fix, but we should seek a better solution that would retain a single source of truth. Maybe in the upgrade scenario we should use release object and set tag on this one, and phase out tag accessor from Docker::Gitlab to rely only on release (and get information from there?).

  What do you think @rymai?

  I'm going to merge this bugfix, and resolve this comment by creating an issue.

~technical debt