Docker: use the buildset registry if defined

If the docker build jobs see that a buildset_registry is defined
(i.e., they are running in a job graph where an earlier job ran
a buildset registry) automatically use that registry.

Change-Id: Ied7730b793a02204921a2e76f8474fcce534ac77

playbooks/docker-image/pre.yaml

@@ -1,3 +1,8 @@
 - hosts: all
   roles:
     - install-docker
+  tasks:
+    - name: Configure buildset registry
+      when: buildset_registry is defined
+      include_role:
+        name: use-buildset-registry