Make sure we're not clobbering another dependency when adding an in-tree docker image (#437)

Author: jcristau

src/taskgraph/transforms/task.py

@@ -364,6 +364,9 @@ def build_docker_worker_payload(config, task, task_def):
         if "in-tree" in image:
             name = image["in-tree"]
             docker_image_task = "build-docker-image-" + image["in-tree"]
+            assert "docker-image" not in task.get(
+                "dependencies", ()
+            ), "docker-image key in dependencies object is reserved"
             task.setdefault("dependencies", {})["docker-image"] = docker_image_task
 
             image = {