Document how to attach to container and test Gnome apps

[olberger]

Hi.

I guess it's possible to connect inside the running container to try and start Gtk apps in the "display", in addition to what's done by init.sh.

It's probably pretty standard docker procedure, but it might be helpful to document that, for people willing to experiment with broadway.

Thanks in advance.

[olberger]

I think the following would do:

docker exec -it gtk3-docker bash