add ci script

.gitlab-ci.yml

+build-docker:
+  # Use the official docker image.
+  image: docker:latest
+  stage: build
+  services:
+    - docker:dind
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  # Default branch leaves tag empty (= latest tag)
+  # All other branches are tagged with the escaped branch name (commit ref slug)
+  script:
+    - |
+      if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        tag=":latest"
+      else
+        tag=":$CI_COMMIT_REF_SLUG"
+      fi
+      echo "Running on branch '$CI_COMMIT_BRANCH': tag = $tag"
+    - docker build --pull -t "$CI_REGISTRY_IMAGE${tag}" .
+    - docker push "$CI_REGISTRY_IMAGE${tag}"