Add zuul labels to images and ability to user-define more

We can attach some metadata to images we build indicating where
the image was built from. We can also allow users to add additional
labels, should they choose, although honestly for users doing it in
the Dockerfile makes more sense.

Change-Id: I01de15279a69026a09633eb488dac62910b324f5

roles/build-docker-image/common.rst

@@ -130,4 +130,10 @@ using this role.
       added with each sibling project.  Note that projects here must
       be listed in ``required-projects``.
 
+   .. zuul:rolevar:: labels
+      :type: list
+      :default: []
+
+      A list of labels to attach to the built image, in the form of "key=value".
+
 .. _anchors: https://yaml.org/spec/1.2/spec.html#&%20anchor//

roles/build-docker-image/tasks/build.yaml

@@ -45,6 +45,11 @@
       {% endif -%}
       --tag {{ item.repository }}:{{ tag }}
     {% endfor -%}
+    {% for label in zj_image.labels | default([]) -%}
+      --label "{{ label }}"
+    {% endfor %}
+    --label "org.zuul-ci.change={{ zuul.change }}"
+    --label "org.zuul-ci.change_url={{ zuul.change_url }}"
   args:
     chdir: "{{ zuul_work_dir }}/{{ item.context }}"