Update .gitlab-ci.yml

.gitlab-ci.yml

 image: docker:19.03.1
+services:
+  - docker:19.03.1-dind
 
 stages:
 - build
 - test
 
+variables:
+  GIT_SUBMODULE_STRATEGY: recursive
+  DOCKER_TLS_CERTDIR: "/certs"
+  DOCKER_HOST: tcp://docker:2376 # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
+
 before_script:
 - docker login -u ${CI_REGISTRY_USER} -p ${CI_REGISTRY_PASSWORD} ${CI_REGISTRY}