Add ceph-container-prerelease-build

Add ceph-container-prerelease-build

Based on ceph-container-build-ceph-base-push-imgs, but
for building containers for testing before the release is
validated and committed.  Differences:

1) pulls packages from download.ceph.com/prerelease
2) does not attempt to build aarch64
3) pushes built container to quay.ceph.io:ceph/prerelease

The prerelease directories are password-protected to avoid any
early exposure to in-progress builds.

This requires corresponding changes to ceph-container.git.

Signed-off-by: Dan Mick <dmick@redhat.com>