Change DOCKER_HOST variable to localhost

.gitlab-ci.yml

@@ -11,7 +11,7 @@ variables:
   # 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/
+  DOCKER_HOST: tcp://localhost:2375/
   # When using dind, it's wise to use the overlayfs driver for
   # improved performance.
   DOCKER_DRIVER: overlay2