Bump to Xpra 2.2.6 and Firefox ESR 52.8.0 and use digest to lock to a…

… specific base image

Ensure the container image is always up-to-date

Bump to Xpra 2.2.4 and Firefox ESR 52.6.0

Mount CUPS Unix socket into container, instead of making it connect v…

…ia TCP

The default configuration for CUPS is to make it listen on localhost -- hence
 we cannot connect from a container that runs on a different IP.