Add the initial .gitlab-ci config.
Makes the necessary changes for the project to be buildable through GitLab CI. The pushing of the Docker image has not yet been tested, but should work in theory
One of the larger changes is that the package name in the source files has been changed from:
github.com/uninett/jwt-tokenissuer to scm.uninett.no/daas/jwt-tokenissuer, to reflect the fact that the package is hosted elsewhere now.