Have aggregate-container-add-tag keep the repository name and just add a tag

[Vogtinator]

Is your feature request related to a problem? Please describe.
For SLE 15 and Leap 15, we have a single container repository on the registry for all versions. The :latest tag points to the latest stable release. e.g. we have opensuse/leap:15.1, opensuse/leap:15.2, opensuse/leap:15.3, and opensuse/leap:latest points to opensuse/leap:15.2 currently. The same mechanism is also used for the :15 tag.

The way this is implemented is by having separate projects for each version and using aggregates to collect them in a project for publishing. Using the prjconf, the aggregate pointing to the latest stable version is assigned the opensuse/leap:latest tag:

Substitute: aggregate-container-add-tag:15.2 opensuse/leap:15 opensuse/leap:latest

Now there are also opensuse/leap-dnf:15.3 and opensuse/leap-microdnf:15.3 images, which need the same treatment. Using the current mechanism this means that for each new name and version a new aggregate and substitute are needed.

Describe the solution you'd like
If OBS could keep the name from the aggregated container image, something like

Substitute: aggregate-container-add-tag:15.2 :15 :latest

could be combined with a single aggregate which contains all of the relevant container images.
This only makes sense for images which have an unambiguous repository name, but that is the case in this case at least.

Describe alternatives you've considered
Any mechanism for assigning tags to built images dynamically should work, even better if it scales better than aggregates, which can be slow sometimes.

Additional context
Can be seen in action here: https://build.opensuse.org/project/show/openSUSE:Containers:Leap