Include .gitlab-ci.yml file

Dear Ed @edward.rial, sometimes life could be easy. We just forgot to create a Merge Request. If there is no merge request, no merge request event is triggered. Sorry for the confusion. Now it should run, create a Docker image in the GitLab Container Registry (https://gitlab.hzdr.de/edward.rial/wRadia/container_registry), and if you are fine with this MR, you can merge it and check that a new Docker image is pushed to Docker Hub (https://hub.docker.com/r/eddrial/wradia/tags). Please let me know if this is working. Thanks!