Update BW Grader Dockerfile

[ezhang887]

Few changes:

• Move the ARG for INSTALL_PATH from before the FROM to after. Previously, broadway was being installed in / rather than /opt/broadway as the Dockerfile intended to. This is what the current BW graders look like:

  ericsz2@fa20-cs241-grd-13:~$ sudo docker exec -it 6925e00437c0 /bin/sh
  / # ls
  bin               home              mnt               root              sys
  broadway          lib               opt               run               tmp
  dev               log               proc              sbin              usr
  etc               media             requirements.txt  srv               var

  "An ARG declared before a FROM is outside of a build stage, so it can’t be used in any instruction after a FROM." from https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact.

• Set the WORKDIR to /srv/cs241/broadway-grader. This way, we can add -v /srv/cs241/broadway-grader:/srv/cs241/broadway-grader as an argument when starting the container which will properly clean up the tmp******** directories in the grader machines.

I've tested these changes on grader 13 a while back. I built a local image using the updated Dockerfile and spun up a BW grader using that, then scheduled a run on grader 13, then verified that the tmp******** directory that was created for the grading pipeline was also cleaned up properly.