ci: add route to index docker-images by PR number

Author: ahal

taskcluster/kinds/docker-image/kind.yml

@@ -10,6 +10,7 @@ loader: taskgraph.loader.transform:loader
 transforms:
     - taskgraph.transforms.docker_image:transforms
     - taskgraph.transforms.cached_tasks:transforms
+    - self_taskgraph.transforms.add_pr_route
     - taskgraph.transforms.task:transforms
 
 # make a task for each docker-image we might want.  For the moment, since we

taskcluster/self_taskgraph/transforms/add_pr_route.py

@@ -0,0 +1,35 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from taskgraph.transforms.base import TransformSequence
+
+transforms = TransformSequence()
+
+
+@transforms.add
+def add_pr_route(config, tasks):
+    """Taskgraph builds several docker-images that get used externally.
+
+    Indexing these images by pull request number, allows us to easily test them
+    in external repos using the `indexed-image` image definition type.
+    """
+    if not (pr_number := config.params.get("pull_request_number")):
+        yield from tasks
+        return
+
+    PR_ROUTE = (
+        "index.{trust-domain}.v2.{project}-pr.{pr-number}.latest.{kind}.{task-name}"
+    )
+    subs = {
+        "trust-domain": config.graph_config["trust-domain"],
+        "project": config.params["project"],
+        "pr-number": pr_number,
+        "kind": config.kind,
+    }
+
+    for task in tasks:
+        subs["task-name"] = task.get("name") or task["label"][len(config.kind)+1:]
+        routes = task.setdefault("routes", [])
+        routes.append(PR_ROUTE.format(**subs))
+        yield task