Fix quadlet parameters when container uses rootfs

[mainnika]

Adjust quadlet parameters to set rootfs as image and clear image field when rootfs container is being used.

e.g.:

  containers.podman.podman_container:
    rootfs: true
    image: "/opt/container-root"

should put into a quadlet file (note, no Image= in a file):

Rootfs=/opt/container-root

[sshnaidm]

Great, thanks for contribution @mainnika !