도커 이미지 자동 빌드 구축

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