Give the container some time to start up

... apparently needed since we launch dbus and sshd before Xpra.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a script to attach to the Docker container running Xpra

This is helpful for debugging, but also to stop the container quickly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use Xpra's APT repository

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>