Standardized Docker tag support

[Nishnha]

Right now there is no standard around versioning for Docker tags, and regex around tags is not fully compatible with semver 2.

Dependabot will currently try to identify dates, build numbers, semver versioning from a Docker tag, which is something that we've settled on through trial and error and bug reports, but we still see feature requests around Docker tags all the time.

So I'd like to ask you all - what should be our standard for Docker tags? Is there already one present that we can adopt?