ceph-dev-pipeline: Build the builder less often

ceph-dev-pipeline: Build the builder less often

To build the builder image less often, we can:

1. When building, use two tags: branch.os.arch, and sha1.branch.os.arch
2. After building, push both tags
3. Before building, pull both tags

As a result, we can avoid building entirely if that exact image has already been
pushed, and avoid building at least some layers if the branch had been built
somewhat recently.

Signed-off-by: Zack Cerza <zack@cerza.org>