The goog spits out many hits for running an X11 program in a container by creating a new container from scratch. Buried in this list was 1 hit for reconfiguring an existing container.
The general idea is to add 1 line to your /var/lib/lxc/*/config file
lxc.mount.entry = /tmp/.X11-unix tmp/.X11-unix none bind,optional,create=dir,ro
Then restart the container.
This creates a path which redirects the X server sockets of display :0 to the X server sockets of the host display.