build: use docker.io/ceph/ceph:v15 as BASE_IMAGE
build: use docker.io/ceph/ceph:v15 as BASE_IMAGE

Use a qualified image name, including the registry where it should come
from. This makes it possible to pull the image from the right location,
and consume it in CI jobs without trying to pull again.

Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi