Add .gitlab-ci.yml

.gitlab-ci.yml

+image: docker:18.09.7
+
+stages:
+  - build
+
+build_mage:
+  stage: build
+  only:
+    - master
+  services:
+    - docker:18.09.7-dind
+  script:
+    - docker build -t $CI_REGISTRY_IMAGE .
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+    - docker push $CI_REGISTRY_IMAGE