Merge branch 'validate_docker_compose' into 'master'

Validate docker-compose.yml

See merge request thalia/concrexit!851

.gitlab-ci.yml

@@ -77,6 +77,7 @@ build docker image:
   before_script:
     - echo $GITLAB_REGISTRY_PASSWORD | docker login --username thaliawww --password-stdin registry.gitlab.com
   script:
+    - docker-compose config -q
     - docker-compose build --build-arg install_dev_requirements=0 web
     - docker-compose run --rm web test
     - docker tag $DOCKER_LATEST $DOCKER_TAG