The experiment of using Nix Shell was not really successful. Most
notably, Nix uses a bunch of wrapper scripts around compilers, which
make the build much less "pure". This may be useful but it's exactly
not what we want for CI. In particular, this resulted in gcc being used
for the "clang" builds because a wrapper script redefined the CC env
variable.
This now builds a single docker image (Debian) for all architectures
that we test in CI on Linux.
