Initial release process for UBI-based CNG images

Summary

Add the UB8 CNG images to our release process

This is a followup issue to #443 (closed) which should be completed first

Also #445 (closed) should be considered for scheduling before this issue.

Also #503 should be completed first.

Proposal

Once we are able to confidently build the UB8 CNG images, and are confident in releasing them to the public, we should determine how best to add them to our current CNG image release process so that they are built when the GitLab release is tagged.

We only want to do this once we are confident these images are ready to be used and don't need to be drastically changed.

References

&4 (closed)