add ci to build pipeline

.gitlab-ci.yml

+docker-build:
+  # 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" == "$CI_DEFAULT_BRANCH" ]]; then
+        tag=""
+        echo "Running on default branch '$CI_DEFAULT_BRANCH': tag = 'latest'"
+      else
+        tag=":$CI_COMMIT_REF_SLUG"
+        echo "Running on branch '$CI_COMMIT_BRANCH': tag = $tag"
+      fi
+    - docker build -t "$CI_REGISTRY_IMAGE${tag}" .
+    - docker push "$CI_REGISTRY_IMAGE${tag}"
+  # Run this job in a branch where a Dockerfile exists
+  rules:
+    - if: $CI_COMMIT_BRANCH
+      exists:
+        - Dockerfile