[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/horizon/Dockerfile b/images/horizon/Dockerfile
index 58e8a65..c119e11 100644
--- a/images/horizon/Dockerfile
+++ b/images/horizon/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:${RELEASE} AS build
 ARG HORIZON_GIT_REF=5de40f9b222608d35c5a0919117259e966217a86
 ADD --keep-git-dir=true https://opendev.org/openstack/horizon.git#${HORIZON_GIT_REF} /src/horizon
 RUN git -C /src/horizon fetch --unshallow
@@ -50,7 +52,7 @@
         /src/senlin-dashboard
 EOF
 
-FROM registry.atmosphere.dev/library/openstack-python-runtime:zed
+FROM registry.atmosphere.dev/library/openstack-python-runtime:${RELEASE}
 RUN <<EOF bash -xe
 apt-get update -qq
 apt-get install -qq -y --no-install-recommends \