Changes

Introduce debian-bullseye-titan-master

This Dockerfile allows building a given repo and branch of
titan.core.git which can then be used by any ttcn-*-test/ environment by
simply changing its Dockerfile "FROM" line from
$REGISTRY/$USER/debian-bullseye-titan to
$REGISTRY/$USER/debian-bullseye-titan-mater.

This is useful to debug or develop titan.core or test whether a given
testsuite works fine with a newer version of titan.core.

Change-Id: I19ee98a319ccad167d06c4f183fe80ecac909483