[stable/zed] Refactor image tags to be dynamic (#1122)

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.
Closes: #1121
diff --git a/images/nova/Dockerfile b/images/nova/Dockerfile
index 7f4a9dd..415034c 100644
--- a/images/nova/Dockerfile
+++ b/images/nova/Dockerfile
@@ -12,7 +12,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-FROM registry.atmosphere.dev/library/openstack-venv-builder:zed AS build
+ARG RELEASE
+
+FROM registry.atmosphere.dev/library/openstack-venv-builder:${RELEASE} AS build
 ARG NOVA_GIT_REF=b4a69447e2a176fd3821abc427019339ec700f0c
 ADD --keep-git-dir=true https://opendev.org/openstack/nova.git#${NOVA_GIT_REF} /src/nova
 RUN git -C /src/nova fetch --unshallow
@@ -22,7 +24,7 @@
         /src/nova
 EOF
 
-FROM registry.atmosphere.dev/library/openstack-python-runtime:zed
+FROM registry.atmosphere.dev/library/openstack-python-runtime:${RELEASE}
 ADD https://github.com/novnc/noVNC.git#v1.4.0 /usr/share/novnc
 RUN <<EOF bash -xe
 apt-get update -qq