Bug 1273926: set attributes of docker-image tasks too; r=ahal

MozReview-Commit-ID: 5lc8481C3Sn

--HG--
extra : rebase_source : 5fa4372e9241c4b73825135973ac111f84cf7480

testing/taskcluster/taskcluster_graph/image_builder.py

@@ -221,6 +221,10 @@ def normalize_image_details(graph, task, seen_images, params, decision_task_id):
                                                    get_json_routes(),
                                                    image_parameters)
 
+    image_task['attributes'] = {
+        'kind': 'legacy',
+    }
+
     graph['tasks'].append(image_task);
     task['requires'].append(details['taskId'])