Use a lighter docker Nix based image for the CI.
Needed some hack on the base Nix image, see https://github.com/LnL7/nix-docker/pull/24
This branch can be used tu build the
Use the cache of gitlab CI for the store.
NOT POSSIBLE because /nix is out of the project scope
Use the scripts in. /ci inside the Nix expressions for tests.
Done in kapack with e74bdf3 and in batsim with 9ed97d4e
use the scheduling capabilities of the gitlab CI to test with upstream Simgrid regularly
DEFERED This is discussed in #90 (closed)
check that Batsim compiles without warning on both clang and gcc.