Bug 1809580 - fix expiration of docker image artifacts r=releng-reviewers,bhearsum DONTBUILD

Set expiration-policy on docker-image tasks to make sure the image artifact
sticks around as long as the task, and avoid issues down the road when we try
to reuse a cached task only to fail when we can't download its artifact.

Differential Revision: https://phabricator.services.mozilla.com/D178289

taskcluster/gecko_taskgraph/transforms/docker_image.py

@@ -132,7 +132,7 @@ def fill_template(config, tasks):
                 "image_name": image_name,
                 "artifact_prefix": "public",
             },
-            "expires-after": "1 year",
+            "expiration-policy": "long",
             "scopes": [],
             "treeherder": {
                 "symbol": job_symbol,