Allow to specify references to OCI containers in the
image description like in the following example:
<containers source="registry.suse.com" backend="podman">
<container name="some" tag="some" path="/some/path"/>
</containers>
During the kiwi process the containers are fetched into a
temporary location and a systemd service is configured to
one time load the containers into the local registry at
first boot of the system. This Fixes#2663
a349c055c4