Open Stack

I’m trying to be able to build a projects container without having to rebuild the parents which have different tags.

The workflow I’m trying to achieve is:

Build base and openstack-base with a tag of wallaby 

Build a container image for barbican with a tag of the version of barbican that is returned when doing `git describe` 
Build a container image for nova with a tag of the version of barbican that is returned when doing `git describe` 
etc.etc.

I don’t seem to be able to do this without having to also build a new base and openstack-base with the same tag which is slow and also means a lot of disk space.

Just wondering how other people do this sort of stuff?
Any ideas?

Thanks,
Sam