Add doxygen + m.css image for C++ documentation
Problem to solve
A docker image that can be used to build C++ project documentation with PDF support and m.css theming is not trivial.
Further details
In particular it is hard to make a small, alpine-based docker image. At the same time, Debian-based images are huge and pulling them takes a lot of time.
Proposal
Add an alpin-based image with doxygen and m.css. Also add an example configuration and publishing job.