Allow specification of image.tagExtension to support easy configuration of UBI container usage
## Summary
Split from #1617, Implement `gitlab.image.tagExtension` to complete support for UBI behaviors. This will need some extensive work to the GitLab maintained charts, to inject a suffix to `image.tag` when populated.
## Details
### Item 5: defining UBI via single flag.
This would not be a tag, but rather an method to automatically append an extension *to the* `tag`, so that when `appVersion: 13.0.1`, the rendered `tag` value is `13.0.1-ubi8`. We should not require *manually* setting every single image tag. It's just an abhorrent user experience.
This could be in Major or Minor, in my opinion.
cc @mendeni @dorrino
issue