chore: bump decision task to modern taskgraph version

The old version is causing issues in mozilla-releng#210, because `get_ancestors` is not properly memoized.

.taskcluster.yml

@@ -237,7 +237,7 @@ tasks:
                               taskclusterProxy: true
                               chainOfTrust: true
                           image:
-                              mozillareleases/taskgraph:decision-v7.3.1@sha256:1d1364c01c13002a13863f0ef27dd7331ad1c7fe92bbb8b0b4019dc5a821461c
+                              mozillareleases/taskgraph:decision-v12.1.0@sha256:756d6edd4c60e1c58132a667273e3cb6113bde3d7a4a3f7b326b375caf53eb81
                           maxRunTime: 1800
                           command:
                               $flatten:

taskcluster/docker/python/Dockerfile

@@ -7,10 +7,10 @@ FROM python:$PYTHON_VERSION
 LABEL maintainer="Mozilla Release Engineering <[email protected]>"
 
 # Add worker user
-RUN mkdir /builds && \
+RUN mkdir -p /builds && \
     useradd -d /builds/worker -s /bin/bash -m worker && \
     mkdir /builds/worker/artifacts && \
-    chown worker:worker /builds/worker/artifacts
+    chown worker:worker /builds/worker /builds/worker/artifacts
 
 # %include-run-task