[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/nova-ssh/Dockerfile b/images/nova-ssh/Dockerfile
index 84f6889..37fc531 100644
--- a/images/nova-ssh/Dockerfile
+++ b/images/nova-ssh/Dockerfile
@@ -12,7 +12,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-FROM registry.atmosphere.dev/library/openstack-runtime:zed
+ARG RELEASE
+
+FROM registry.atmosphere.dev/library/openstack-runtime:${RELEASE}
 RUN <<EOF bash -xe
 apt-get update -qq
 apt-get install -qq -y --no-install-recommends \