Thank you Jaime, that hit the spot. I summarize the steps I adopted from this excellent post:
1. In the docker run command line include -v /tmp/.X11-unix:/tmp/.X11-unix:ro to couple the working directories of the x server in the container and in the OS.
2. In the docker run command line include -e DISPLAY=$DISPLAY to set the container's DISPLAY env var to the same value as the OS.
3. In the OS do xhost +local:root to allow the container access to the OS's x-server.
See important comments in the post (
http://stackoverflow.com/a/25334301/255832) about
security while working in a server or on a machine with multiple users. You will then need to set the container to run as a user (not as root) and set xhost to allow access only to that user.
Thanks again,
Amir