The rpm_docker test was updated to include RHEL iso creation.
It works but it would be better if you could mount the simpcore directory and build in there.
I started work on this but ran into a problem with not GPG needing to create hard links on the file system to create a lock.
My changes so far are in https://github.com/jeannegreulich/simp-core/tree/SIMP-6986-buildrh-mounting-simp-core
These articles might help:
https://github.com/hashicorp/vagrant/issues/713 but I am not sure if they are talking about creating hardlinks on a mounted file system or linking to the mounted file system.