Check build dependencies used by gitian build

[cornelius]

The gitian build script contrib/gitian-build.py does the release builds of unit-e. It has a number of direct and indirect dependencies which affect the build. We need to check these dependencies and make sure that we are using the versions we want to use and we don't miss for example relevant security updates.

Dependencies are:

• The gitian builder, this is pulled in via git from the master branch. We probably should rather use a fixed revision.
• The packages used to build the gitian base container
• The packages pulled into the container for doing the build
• The unit-e dependencies from the depends directory
• The unit-e dependencies pulled in via git-subtree
• The dependencies downloaded at runtime of the gitian-build.py script
• The packages installed by the gitian-build.py script