Require privileged mode for docker in docker

.gitlab-ci.yml

@@ -9,7 +9,7 @@ services:
 
 build-ubuntu-18.04:
   stage: build
-  tags: [ linux, docker ]
+  tags: [ linux, docker, privileged ]
   image: docker:latest
   before_script:
     - echo "${CI_REGISTRY_PASSWORD}" | docker login -u "${CI_REGISTRY_USER}" "${CI_REGISTRY}" --password-stdin