Create Docker containers for Nexus and Reprepro

Nexus is a Maven repository and Reprepro is a Debian repository.

The purpose is twofold:
1. to demonstrate/document how to setup these services and to test them, and
2. to be able to use the services for making artifacts/packages
   available to tests inside a guest box.

A possible advantage of the latter is that untested artifacts/packages
don't necessarily have to be build locally in the test box, or deployed
to a public repository.