Bug 1655893 - docker_image.py transform should set `artifact_prefix`; r=tomprince

Differential Revision: https://phabricator.services.mozilla.com/D85232

taskcluster/taskgraph/transforms/docker_image.py

@@ -137,7 +137,10 @@ def fill_template(config, tasks):
         taskdesc = {
             'label': 'build-docker-image-' + image_name,
             'description': description,
-            'attributes': {'image_name': image_name},
+            'attributes': {
+                'image_name': image_name,
+                'artifact_prefix': 'public',
+            },
             'expires-after': '28 days' if config.params.is_try() else '1 year',
             'scopes': [],
             'treeherder': {