Try something

Signed-off-by: Adrian C. Hinrichs <adrian.hinrichs@rwth-aachen.de>

.gitlab-ci.yml

 stages:
   - build
+variables:
+  # When using dind service we need to instruct docker, to talk with the
+  # daemon started inside of the service. The daemon is available with
+  # a network connection instead of the default /var/run/docker.sock socket.
+  #
+  # The 'docker' hostname is the alias of the service container as described at
+  # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services
+  #
+  # Note that if you're using Kubernetes executor, the variable should be set to
+  # tcp://localhost:2375 because of how Kubernetes executor connects services
+  # to the job container
+  DOCKER_HOST: tcp://docker:2375/
+  # When using dind, it's wise to use the overlayfs driver for
+  # improved performance.
+  DOCKER_DRIVER: overlay2
 
 build testing:
   image: docker:stable-dind