Support mixed (Linux+Windows) Docker images
Docker for Windows supports running mixed (Linux+Windows) Docker images, see https://devblogs.microsoft.com/premier-developer/mixing-windows-and-linux-containers-with-docker-compose/
but GitLab Runner does not allow that due f1ce81ca .
Steps to reproduce
Start any Windows job with some Linux service.
Running with gitlab-runner 11.11.2 (ac2a293c) on ...removed... ERROR: Job failed: executor requires OSType=linux, but Docker Engine supports only OSType=windows
Linux service/image is started on Windows host.