Use overlay2 storage driver for better performance than default vfs

.gitlab-ci.yml

@@ -3,6 +3,8 @@ image: docker:19.03.0
 # disable TLS just for the docker daemon running locally, TLS is still used to deploy built images!
 variables:
     DOCKER_TLS_CERTDIR: ""
+    DOCKER_DRIVER: overlay2    
+
 services:
     - docker:19.03.0-dind