Docker is an awesome tool to run virtual machines without overhead. Docker containers start in 2 seconds and use less memory and disk space. One requirements is that the host needs to be a Linux (virtual) machine.
If you use GitLab CI to run your tests you have one or more runners performing the tests. More runners are better because they enable multiple tests running in parallel. With the new Dockerfile for GitLab CI runner you can add runners with less overhead. Enjoy!