Run GUI application in MacOS 10 in Docker
I was not using the right display number. The guide above uses the default display number :0. 

Find the correct one by running:

  display_number=`ps -ef | grep "Xquartz :\d" | grep -v xinit | awk '{ print $9; }'`


Then change your docker run command to include the flags:

-e DISPLAY=$ip:$display_number \
-v /tmp/.X11-unix:/tmp/.X11-unix \