This removes the need to copy all the data into the docker container
and outputs from the build process out of the container. In
hindsight, this was a poor design decision as it created docker
images that were very large with data, that you don't have to
keep stored once the build finished, e.g. the build output.
Therefore, mount the local folder, keep the checkouts also local
which is good for debugging and also re-using them to build without
the docker container (no second checkout required).
Build output is also written directly to the build/ output folder
Overall, the docker container is more lightweight and actually
a "build environment" only not tainted with specific versions
of the dependencies to build.
Keep normal/dev builds (e.g. just run make) without this flag
to allow keeping warnings around and not being forced to clean
them up during development.
