Update .gitlab-ci.yml

.gitlab-ci.yml

+variables:
+  DOCKER_TLS_CERTDIR: ""
+docker-build-master:
+  # Official docker image.
+  image: docker:latest
+  stage: build
+  services:
+    - docker:dind
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  script:
+    - docker build --pull -t "$CI_REGISTRY_IMAGE":prod .
+    - docker push "$CI_REGISTRY_IMAGE":prod