Build container images using pbrx

The pbrx project has a command to build container images based on the
python:alpine base image for any project using pbr and bindep.

Use it to start building container images for zuul. A followup patch
will add support for publishing the images to dockerhub.

The images want the database drivers in them to be useful out of the
box. Also, the executor wants ara installed. By adding those to extra
entries named after the resulting image name, pbrx will install the
extra packages at the right time, as well as produce wheels for them
during the wheel cache stage.

Depends-On: https://review.openstack.org/583047
Change-Id: I453b0ab2c4916cb77249b5f1039e6e18fde47ab1

.zuul.yaml

@@ -98,6 +98,7 @@
               - yarn.lock
         - zuul-stream-functional
         - zuul-tox-remote
+        - pbrx-build-container-images
         - nodepool-zuul-functional:
             voting: false
     gate:
@@ -132,6 +133,7 @@
               - yarn.lock
         - zuul-stream-functional
         - zuul-tox-remote
+        - pbrx-build-container-images
     post:
       jobs:
         - publish-zuul-docs

setup.cfg

@@ -44,3 +44,13 @@ mysql_reporter=
     PyMySQL
 migrate=
     jenkins-job-builder==1.6.2
+# The container image build process will look at the python extras and install
+# any that match the target container name. It makes a container image for
+# every console_script listed above. The extra name must have underscores.
+# This is a place to put dependencies that should go into reasonable container
+# images but that we do not want to list directly in requirements.txt.
+zuul_base =
+    PyMySQL
+    psycopg2-binary
+zuul_executor =
+    ara