Refactor image tags to be dynamic (#1120)

In order to simplify backporting and branching of new releases,
this patch relies on the Zuul branch when building images so
that we can easily branch out a new release and it will
automatically pick up the images.
diff --git a/images/python-base/Dockerfile b/images/python-base/Dockerfile
index c098401..883a4f4 100644
--- a/images/python-base/Dockerfile
+++ b/images/python-base/Dockerfile
@@ -12,7 +12,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-FROM registry.atmosphere.dev/library/ubuntu-cloud-archive:main
+ARG RELEASE
+
+FROM registry.atmosphere.dev/library/ubuntu-cloud-archive:${RELEASE}
 ENV PATH=/var/lib/openstack/bin:$PATH
 RUN \
     apt-get update -qq && \