Dockerfile: Remove an unused CEPH_TAG env variable
Dockerfile: Remove an unused CEPH_TAG env variable

Ceph version is already available from CEPH_VERSION env variable.
CEPH_TAG on the other hand is only defined to determine the tag for
ceph base image and seems to be unused in the resulting go-ceph
container image.

Signed-off-by: Anoop C S <52add29bc15c0f3832a373982b132796d33ff6c3@cryptolab.net>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph