Debian 9.0 ships with titan 6.1.0 which for some unknown reasons
causes segfaults in the C++ part when parsing CTRL messages and
according to TITAN project is not supported anymore anyway.
So let's use a 6.3.x build, or whatever is the highest version in
the network:osmocom:latest feed instead.
A lof of what the Dockerfiles so far did in terms of cloning library
repositories and calling shell scripts has now been implemented as
part of the Makefiles inside osmo-ttcn3-hacks.git, so we can drop it
here and simply use those.