[hoherd]

It sounds like you missed the x11vnc part of this. You connect into the container using vnc, not x11, and potentially via a browser based vnc client. If you want to see a working example of a gui app running successfully in docker, check out https://github.com/jlesage/docker-handbrake

[FPGAhacker]

the vnc solution was one of several presented in the article. The second was using the host x11 server by sharing the socket.