Considering adding the dependencies pybind11, googletest and spdlog in the container directly.

Currently, we are downloading the dependencies using FetchContent_Declare() during the cmake process. This works quite well in the build process and is well maintainable, as the version or the source-path can be changed easily. The disadvantage is, that the cmake process in general takes longer. Also it happens from time to time that the download failed, which is a bit annoying. That's why I would consider whether it is worth providing these packages with the docker image.