[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/placement/Dockerfile b/images/placement/Dockerfile
index 2aeaf23..0cfa6b5 100644
--- a/images/placement/Dockerfile
+++ b/images/placement/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:${RELEASE} AS build
 ARG PLACEMENT_GIT_REF=d7ced6bd2fc82caf458f20b5652888164b1bbb70
 ADD --keep-git-dir=true https://opendev.org/openstack/placement.git#${PLACEMENT_GIT_REF} /src/placement
 RUN git -C /src/placement fetch --unshallow
@@ -22,5 +24,5 @@
         /src/placement
 EOF
 
-FROM registry.atmosphere.dev/library/openstack-python-runtime:zed
+FROM registry.atmosphere.dev/library/openstack-python-runtime:${RELEASE}
 COPY --from=build --link /var/lib/openstack /var/lib/openstack