[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/magnum/Dockerfile b/images/magnum/Dockerfile
index f945cf1..103adfe 100644
--- a/images/magnum/Dockerfile
+++ b/images/magnum/Dockerfile
@@ -12,7 +12,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-FROM registry.atmosphere.dev/library/ubuntu:zed AS helm
+ARG RELEASE
+
+FROM registry.atmosphere.dev/library/ubuntu:${RELEASE} AS helm
 ARG TARGETOS
 ARG TARGETARCH
 ARG HELM_VERSION=3.14.0
@@ -20,7 +22,7 @@
 RUN tar -xzf /helm.tar.gz
 RUN mv /${TARGETOS}-${TARGETARCH}/helm /usr/bin/helm
 
-FROM registry.atmosphere.dev/library/openstack-venv-builder:zed AS build
+FROM registry.atmosphere.dev/library/openstack-venv-builder:${RELEASE} AS build
 ARG MAGNUM_GIT_REF=0ee979099a01ae2c8b1b5d6757897a8993e4e34c
 ADD --keep-git-dir=true https://opendev.org/openstack/magnum.git#${MAGNUM_GIT_REF} /src/magnum
 RUN git -C /src/magnum fetch --unshallow
@@ -33,7 +35,7 @@
         magnum-cluster-api==0.16.0
 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 \