Fix and test multiarch docker builds in a release pipeline

This corrects the following error when upload-docker-image is
used in a release pipeline:

  The task includes an option with an undefined variable.
  The error was: 'dict object' has no attribute 'change'

Change-Id: I70cd566ddd0b931d635b20107d652f3591a133b6

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

@@ -23,7 +23,9 @@
       {% for label in zj_image.labels | default([]) -%}
         --label "{{ label }}"
       {% endfor %}
-      --label "org.zuul-ci.change={{ zuul.change }}"
+      {% if zuul.change | default(false) -%}
+        --label "org.zuul-ci.change={{ zuul.change }}"
+      {% endif -%}
       --label "org.zuul-ci.change_url={{ zuul.change_url }}"
 
 - name: Build images for all arches

zuul-tests.d/container-roles-jobs.yaml

@@ -86,6 +86,19 @@
         - name: builder
           label: ubuntu-bionic
 
+- job:
+    name: zuul-jobs-test-build-docker-image-release-multiarch
+    parent: zuul-jobs-test-build-docker-image-release
+    description: |
+      Test building a docker image in a release pipeline.
+
+      This job tests changes to the build-docker-image role using buildx,
+      simulating its use within a tag-based release pipeline.  It is
+      not meant to be used directly but rather run on changes to roles
+      in the zuul-jobs repo.
+    vars:
+      multiarch: true
+
 - job:
     name: zuul-jobs-test-registry-docker
     description: |
@@ -378,6 +391,7 @@
         - zuul-jobs-test-ensure-docker-ubuntu-bionic
         - zuul-jobs-test-ensure-docker-ubuntu-xenial
         - zuul-jobs-test-build-docker-image-release
+        - zuul-jobs-test-build-docker-image-release-multiarch
         - zuul-jobs-test-registry-docker
         - zuul-jobs-test-registry-docker-multiarch
         - zuul-jobs-test-registry-podman