Image tags are not are versions and shouldn't be treated as such. The tag "19.3.6" refers to the version of Gitlab, which hasn't changed. This is why everyone should pin their images to a specific sha and setup a process to get notified on changes
that's incredibly confusing.
what's the point of the tag if the version is really the sha it happens to now point to?
it only looks like semver?
Using Renovate can be one of those processes.