Separate docker files for testing

The Linux builds are done inside a dedicated build container. The tests used to be run in a nearly identical container where the source code had been removed and the build output had been added.

The consequence of that is that the "test" container had a lot more dependencies installed than what it needs at runtime. This also means that there was no documentation on what the minimal list of dependencies is.

This merge request adds proper dockerfiles for setting up an image with OpenZGY/Python and OpenZGY/C++ installed and nothing else. That takes care of both the documentation and of testing that the listed dependencies are sufficient.

Also include an unrelated change to a Makefile to make smaller binaries.