Add CI config.

.gitlab-ci.yml

+image: docker:latest
+
+services:
+  - docker:dind
+
+before_script:
+  - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+
+build-backend:
+  stage: build
+  script:
+    - docker build --pull -f _deployment/Dockerfile.node -t "$CI_REGISTRY_IMAGE/backend" .
+    - docker push "$CI_REGISTRY_IMAGE/backend"
+  only:
+    - main
+
+build-nginx:
+  stage: build
+  script:
+    - docker build --pull -f _deployment/Dockerfile.nginx -t "$CI_REGISTRY_IMAGE/nginx" .
+    - docker push "$CI_REGISTRY_IMAGE/nginx"
+  only:
+    - main