Test network playing in the frame of GL-CI smoke tests

[LynxAbraxas]

It should be possible to set up two docker containers for testing some of the basics for network playing in the frame of the GL-CI smoke tests (introduced with #84). This would be especially interesting for testing game concepts that involve more than one player, like agreements, treaties, pirating and their messages.
This setup could also be used to manually test playing over a "fake-remote" network locally.