Add job to build container images of zuul

This way we can ensure that we don't break zuul with pbrx changes.

Depends-On: https://review.openstack.org/582276
Change-Id: I99c23b41f339708508a2cb6a01801d00b3c7f3cf

.zuul.yaml

@@ -6,6 +6,14 @@
     pre-run: playbooks/container-images/pre.yaml
     run: playbooks/container-images/run.yaml
 
+- job:
+    name: pbrx-build-zuul-containers
+    parent: pbrx-build-container-images
+    required-projects:
+      - openstack-infra/zuul
+    vars:
+      zuul_work_dir: src/git.openstack.org/openstack-infra/zuul
+
 - project:
     check:
       jobs:
@@ -16,6 +24,7 @@
         - build-sphinx-docs:
             vars:
               sphinx_python: python3
+        - pbrx-build-zuul-containers
     gate:
       jobs:
         - tox-pep8
@@ -25,3 +34,4 @@
         - build-sphinx-docs:
             vars:
               sphinx_python: python3
+        - pbrx-build-zuul-containers