Add docker gitlab ci

This MR will close #15 (closed). This is a new Gitlab CI system that makes use of Docker. The reasons for switching over are as follows,

• Docker containers are not persistent, so builds are always clean
• Very easy to set up Gitlab runner for people who want to be part of the pool
• With the Docker image, everyone will have the exact same environment, which lowers the number of possible issues

Also, this MR cleans up a lot of the gitlab-ci.yaml file, and changes how some of it works,

• Nightly release executables are now going to be built every morning at 06:00 GMT
• Debug executables are created for every commit that goes to master, or is part of a MR