Update docker image definitions for compatibility with taskgraph 10.x

The newer image_builder creates directories listed as volumes before
running any other commands.

taskcluster/docker/linux/Dockerfile

@@ -4,9 +4,12 @@
 
 FROM ubuntu:22.04
 
+VOLUME /builds/worker/checkouts
+VOLUME /builds/worker/.cache
+
 # Add worker user
 
-RUN mkdir /builds && \
+RUN mkdir -p /builds && \
     useradd -d /builds/worker -s /bin/bash -m worker && \
     chown worker:worker /builds/worker && \
     mkdir /builds/worker/artifacts && \
@@ -175,9 +178,6 @@ ENV SHELL=/bin/bash \
     HOME=/builds/worker \
     PATH=/builds/worker/.local/bin:$PATH
 
-VOLUME /builds/worker/checkouts
-VOLUME /builds/worker/.cache
-
 # Switch to the worker user to install rustup
 # After startup, run-task will downgrade it's permissions to be this user
 USER worker

taskcluster/docker/linux2004/Dockerfile

@@ -8,9 +8,12 @@
 
 FROM ubuntu:20.04
 
+VOLUME /builds/worker/checkouts
+VOLUME /builds/worker/.cache
+
 # Add worker user
 
-RUN mkdir /builds && \
+RUN mkdir -p /builds && \
     useradd -d /builds/worker -s /bin/bash -m worker && \
     chown worker:worker /builds/worker && \
     mkdir /builds/worker/artifacts && \
@@ -83,8 +86,5 @@ ENV SHELL=/bin/bash \
     HOME=/builds/worker \
     PATH=/builds/worker/.local/bin:$PATH
 
-VOLUME /builds/worker/checkouts
-VOLUME /builds/worker/.cache
-
 # run-task needs to run as root (after initialization, it changes to `worker`)
 USER root