Add USER to the docker environment

Apparently Jack needs this, and my previous testing didn't show it.

Test: build & run image; echo $USER
Change-Id: I66766b230f2f3e0762a49efc9bf2c6212b2b2c4d

tools/docker/Dockerfile

@@ -21,5 +21,6 @@ RUN groupadd -g $groupid $username \
 COPY gitconfig /home/$username/.gitconfig
 RUN chown $userid:$groupid /home/$username/.gitconfig
 ENV HOME=/home/$username
+ENV USER=$username
 
 ENTRYPOINT chroot --userspec=$(cat /root/username):$(cat /root/username) / /bin/bash -i