Add upload-docker-image release job

This will allow us to publish container images to docker when we git
tag. Also add tags field to image_vars, like we do for zuul and nodepool
images.

Change-Id: I9d4cde7a15c6bc57de62ebe3dcafe209e147aefc
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

.zuul.yaml

@@ -28,6 +28,10 @@
         - context: .
           repository: zuul/zuul-registry
           target: zuul-registry
+          tags:
+            # If zuul.tag is defined: [ '3', '3.19', '3.19.0' ].  Only works for 3-component tags.
+            # Otherwise: ['latest']
+            &imagetag "{{ zuul.tag is defined | ternary([zuul.get('tag', '').split('.')[0], '.'.join(zuul.get('tag', '').split('.')[:2]), zuul.get('tag', '')], ['latest']) }}"
 
 - job:
     name: zuul-registry-upload-image
@@ -80,3 +84,11 @@
     release:
       jobs:
         - opendev-release-python
+        - upload-docker-image:
+            secrets:
+              name: docker_credentials
+              secret: zuul-registry-dockerhub
+              pass-to-parent: true
+            vars:
+              <<: *image_vars
+              upload_docker_image_promote: false