Docker images for Linux builds

We should migrate our linux builds to docker images. It will give us better flexibility, simpler builds and re-usable wheels across distributions. Everything we need from the system admins is in place. The next big job is to build the base docker image, which should be based on an old Cent OS 5 distribution (for compatibility):

• Base docker container: https://github.com/pypa/manylinux
• Instructions for the runners: http://docs.gitlab.com/ce/ci/docker/using_docker_images.html

On the top of that, we should install our Conda devevelopment environment as from snippet $6.