Add docker build output to task result (#805)

The build output is only added during failures, but its useful to
have this available during normal task execution as well.

tests/integration/targets/docker_image/tasks/tests/docker_image.yml

@@ -136,6 +136,11 @@
     - repository_1 is changed
     - repository_2 is not changed
 
+# Uncomment in community.docker
+# - assert:
+#     that:
+#       - 'FROM busybox' in repository_1.stdout
+
 - name: Get facts of image
   docker_image_info:
     name: "{{ test_image_base }}:latest"