handbook: add required domaintools python module

[Villemoes]

I've started defining a custom "domain" allowing us to define various
syntactic elements, which is nice both for syntax highlighting, but also
for creating an index of e.g. common variables. But this requires a few
python modules provided via pip.

This also combines the apt-get update with the apt-get install commands,
which is apparently necessary and a common
gotcha (https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/).

[esben]

While at it, you should also combine the
rm -rf /var/lib/apt/lists/*
command to the apt-get commands. The apt-get clean should according to the link above not be needed.