Add .gitlab-ci.yml

The build currently fails, but that does look like a real error and not a bug in the CI scripts.

Yes, that's what it looks like.

mentioned in commit 7d20df9a

Status changed to merged

Do you mind setting CMAKE_C_COMPILER to clang in case you set the C++ compiler to clang++? I don't know whether it has any negative effect otherwise, but I'd prefer it that way.

mentioned in merge request !113 (closed)

@gruenich I updated opts.clang to also set CMAKE_C_COMPILER. It shouldn't affect anything.

I still need to move the Git repository for the Docker stuff somewhere accessible... Maybe a new infrastructure/duneci-docker project? (Currently it can be found on https://github.com/aburch/dune-docker, but needs some cleanup.)

Mentioned in commit 7d20df9a