Build container to generate overrides for an rpm-ostree based system
Goal: support taking a set of RPMs as well as an arbitrary set of files in rootfs/overlays
(which we turn into an RPM) and using this builder container, generate
an output container that can be applied by the https://github.com/openshift/machine-config-operator/
Accepted inputs:
rootfs/overlay: Arbitrary filesystem tree.
rpms/overlay: Passed to rpm-ostree install
rpms/overrides: Passed to rpm-ostree override replace