[draft as some unit tests are missing and at least one fixup! commit needs the right place]

This PR adds support for building images without root or a privileged container using the "supermin" binary. It will require access to /dev/kvm though to make the builds reasonably fast.

Kudos to cgwalters and jlebon for the excellent help and examples in https://github.com/osbuild/bootc-image-builder/issues/98

/jira-epic COMPOSER-2487