Introduce container_registry for image prefix

Change-Id: I80067f53aa2b63286b654fff782f0f7b65a2d050
diff --git a/zuul.d/container-images/heat.yaml b/zuul.d/container-images/heat.yaml
index 4c2fd87..98b3d2c 100644
--- a/zuul.d/container-images/heat.yaml
+++ b/zuul.d/container-images/heat.yaml
@@ -44,10 +44,11 @@
       container_images:
         - context: images/heat
           registry: harbor.atmosphere.dev
-          repository: harbor.atmosphere.dev/library/heat
+          repository: "{{ container_registry }}/heat"
           arch:
             - linux/amd64
           build_args:
+            - REGISTRY={{ container_registry }}
             - "RELEASE={{ zuul.branch | replace('stable/', '') }}"
             - PROJECT=heat
           tags: