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/keystone/Dockerfile b/images/keystone/Dockerfile
index a589dca..29bd6fb 100644
--- a/images/keystone/Dockerfile
+++ b/images/keystone/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:main AS build
+ARG RELEASE
+
+FROM registry.atmosphere.dev/library/openstack-venv-builder:${RELEASE} AS build
 ARG KEYSTONE_GIT_REF=8ca73f758bb613a57815fbe4ae78e3d2afa4af49
 ADD --keep-git-dir=true https://opendev.org/openstack/keystone.git#${KEYSTONE_GIT_REF} /src/keystone
 RUN git -C /src/keystone fetch --unshallow
@@ -23,7 +25,7 @@
         keystone-keycloak-backend==0.1.8
 EOF
 
-FROM registry.atmosphere.dev/library/openstack-python-runtime:main
+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 \