Improvements based on usage at OSC

[treydock]

So far the changes I've used at Ohio Supercomputer Center to build images in an attempt to replicate what we do with NFS root.

• EL9 image-builder image in order to build RHEL 9.4 images. Fixes [BUG] RPM database issue building RHEL 9.4 images #33
• Support args with cmd so that I can do things like mount SSSD socket during image build.
• Document --userns flag that allows the container's builder user to access things that might be 0600 permissions like locally stored secrets mounted into the build.

[travisbcotton]

These all look perfectly reasonable to me. Appreciate the contributions!

[treydock]

Added commit to fix #36

Publishing to registry at OMIT/image-builder
pushing layer pitzer-compute to OMIT/image-builder/pitzer-compute:9.4
pushing layer pitzer-compute to OMIT/image-builder/pitzer-compute:latest
pushing layer pitzer-compute to OMIT/image-builder/pitzer-compute:20250817

[treydock]

The last commit makes it so there is one buildah commit for registry push but multiple buildah tag commands to associate the other tags with the commit and push those tags to registry using same image SHA.