src/script: rename CEPH_BRANCH to CEPH_BASE_BRANCH for build container

src/script: rename CEPH_BRANCH to CEPH_BASE_BRANCH for build container

Previously, we were passing build argument of CEPH_BRANCH, but that was
a bit misleading as we expect the current branch to vary a bit (as users
will be using branches to develop and test the code). What we actually
care about is the base branch ('main', 'squid', etc) as that is fed into
our bootstrap script and we want the option to simple variations based
on the name of said base branch.
Rename CEPH_BRANCH to CEPH_BASE_BRANCH for clarity.

Signed-off-by: John Mulligan <jmulligan@redhat.com>