Use fixuid to resolve user perm conflicts in mounted dirs

It matches the user permissions in the container to those in the host. This has
clear advantages to our previous solution, which matched the permissions in the
image. One immediate benefit is that the jenkins server-build node can pull the
image from Docker Hub, rather than build it on the host (for each build, to
ensure the image is up-to-date).

Dockerfile

@@ -296,10 +296,20 @@ RUN \
 ########################
 
 ARG docker_user
-ARG docker_user_id
 ARG docker_password
 
 RUN \
-    useradd -u $docker_user_id -m $docker_user \
-    && echo "${docker_user}:${docker_password}" \
-    | chpasswd && adduser $docker_user sudo
+    useradd -m $docker_user \
+    && echo "${docker_user}:${docker_password}" | chpasswd \
+    && adduser $docker_user sudo
+
+RUN USER=$docker_user && \
+    GROUP=$docker_user && \
+    curl -SsL https://github.com/boxboat/fixuid/releases/download/v0.5.1/fixuid-0.5.1-linux-amd64.tar.gz | tar -C /usr/local/bin -xzf - && \
+    chown root:root /usr/local/bin/fixuid && \
+    chmod 4755 /usr/local/bin/fixuid && \
+    mkdir -p /etc/fixuid && \
+    printf "user: $USER\ngroup: $GROUP\n" > /etc/fixuid/config.yml
+
+USER $docker_user:$docker_user
+ENTRYPOINT [fixuid]