set docker host var

Docker/Linux_Conan/.gitlab-ci.yml

@@ -9,6 +9,11 @@ docker_linux:
     - name: docker:dind
       entrypoint: ["env", "-u", "DOCKER_HOST"]
       command: ["dockerd-entrypoint.sh"]
+  variables:
+    DOCKER_HOST: tcp://docker:2375/
+    DOCKER_DRIVER: overlay2
+    # See https://github.com/docker-library/docker/pull/166
+    DOCKER_TLS_CERTDIR: ""
   script:
     - docker build -t test Docker/Linux_Conan
   only: