Bug 1429954 - Limit docker images retention on try. r=dustin

taskcluster/taskgraph/transforms/task.py sets an expiry to 28 days for
tasks on try, vs. 1 year on other projects, but only do that when an
expiry is not already set, which docker images do. And they do always
set to 1 year.

But it doesn't make sense to keep large docker images from try for a
year. So use the same retention policy as the default one. We /could/
just remove the expiry from docker images and get the task.py default,
but it seems like whatever future change might happen to that default
shouldn't affect docker images, so it's better to duplicate the setting.

taskcluster/taskgraph/transforms/docker_image.py

@@ -113,7 +113,7 @@ def fill_template(config, tasks):
             'label': 'build-docker-image-' + image_name,
             'description': description,
             'attributes': {'image_name': image_name},
-            'expires-after': '1 year',
+            'expires-after': '28 days' if config.params['project'] == 'try' else '1 year',
             'scopes': ['secrets:get:project/taskcluster/gecko/hgfingerprint'],
             'treeherder': {
                 'symbol': job_symbol,