Export X socket as a VOLUME to allow launching containers on its DISPLAY

[olberger]

We've been inspired by what https://github.com/JAremko/docker-x11-bridge is doing for an Xpra wrapper, which allows to run an X client inside a container on the Xpra bridge's X server.

The same can be acomplished quite easily for noVNC once the /tmp/.X11-unix is shared as a VOLUME, which allows doing things like:

• make run
• docker run --rm --name firefox --volumes-from ubuntu-desktop-lxde-test -e DISPLAY=:1 jess/firefox

See the corresponding trivial VOLUME declaration in: olberger@b4e061d

Hope this makes sense