add .gitlab-ci.yml

.gitlab-ci.yml

+variables:
+  IMAGE_TAG: $CI_REGISTRY_IMAGE/jupyter:$CI_COMMIT_REF_NAME
+  RELEASE_TAG: $CI_REGISTRY_IMAGE/jupyter:latest
+  BUILD_IMAGE: $CI_REGISTRY_IMAGE/docker-git
+
+stages:
+  - build
+  - release
+
+
+.prepare_docker:
+  before_script: &prepare_docker |
+    docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
+
+
+build_docker_image:
+  stage: build
+
+  image: $BUILD_IMAGE
+
+  before_script:
+    - *prepare_docker
+
+  script:
+    - docker build -t $IMAGE_TAG .
+    - docker push $IMAGE_TAG
+
+
+release_docker_image:
+  stage: release
+
+  image: $BUILD_IMAGE
+
+  before_script:
+    - *prepare_docker
+
+  script:
+    - docker pull $IMAGE_TAG
+    - docker tag $IMAGE_TAG $RELEASE_TAG
+    - docker push $RELEASE_TAG
+
+  dependencies:
+    - build_docker_image
+
+  only:
+    - master