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/neutron/Dockerfile b/images/neutron/Dockerfile
index 716aa1d..d10ac94 100644
--- a/images/neutron/Dockerfile
+++ b/images/neutron/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:${RELEASE} AS build
 ARG NEUTRON_GIT_REF=019294c71d94b788c14b23dc1da3c21f51bcdb0b
 ADD --keep-git-dir=true https://opendev.org/openstack/neutron.git#${NEUTRON_GIT_REF} /src/neutron
 RUN git -C /src/neutron fetch --unshallow
@@ -25,7 +27,7 @@
         /src/neutron-vpnaas
 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 \