Add .gitlab-ci.yml

.gitlab-ci.yml

+image: docker:19.03.1
+
+variables:
+  DOCKER_TLS_CERTDIR: "/certs"
+  IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG
+
+services:
+  - docker:19.03.1-dind
+
+before_script:
+  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+
+build:
+  stage: build
+  script:
+    - docker build -t $IMAGE_TAG .
+    - docker push $IMAGE_TAG