(Update 26/11/13) Docker 0.6.5 has a breaking change with ports and exposed. It does not provide a port number when you run sudo docker port $CONTAINER_ID 22 anymore. You can assign the port you expose when you do the docker run command as shown below.
To get ubuntu 12.04 to connect to the xpra on the docker-desktop you need to do a few things to get a newer version of Xpra to install.
Ubuntu does not give you an updated version on its packages it knows about so you have to include a new apt-get repo to find it.
Docker-desktop uses ubuntu 12.10 which has xpra version 0.3.11. This is acting as the server.
On your ubuntu 12.04(client) has xpra version 0.0.7.36 will never connect properly to xpra(sever) 0.3.11 it will seem to connect but never log any information about what is going wrong and what is not working.
The 12.04(client) needs to update to xpra to version 0.9.8-1. I have not gotten versions above 0.9.8-1 to work with xpra 0.3.11(server), tt has problems with rendering the screen.