container: add label ceph=True back

container: add label ceph=True back

Add a label used by cephadm internally that was always set by
ceph-container [1] back to the new containerfile. This should
prevent issues with cephadm shell command thinking official ceph images
are not official ceph images.

[1] https://github.com/ceph/ceph-container/blob/30dc8b9a55f70a40983fd2da6ac31e1b9e977143/src/__DOCKERFILE_TRACEABILITY_LABELS__#L5

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit b4f4323e6ccacbff8899ef9b5c336b238101fe0e)