Move unit test environment from dedicated runner to generic runner using docker

Up to now a dedicated gitlab-runner was used to do CI in RAMSIS. All dependencies had to be set up statically in the runner VM. With a generic docker based runner that sets up dependencies on-the-fly we'll be able

• to adjust our dependencies in requirements.txt whenever we want and the test machine will reflect that
• to improve reproducibility by executing every test run in a pristine environment
• to more easily switch to a different CI environment if we want to (e.g. Jenkins)