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 🙏 (that part uses basically the same config as daas/appstore-api).

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.