CI: Make unnecessary nginx build manual to speedup builds

.gitlab-ci.yml

@@ -20,6 +20,7 @@ build_app:
 
 build_nginx:
   stage: build
+  when: manual
   script:
     - docker pull $CI_REGISTRY_IMAGE:master-nginx || true
     - docker build --cache-from $CI_REGISTRY_IMAGE:master-nginx -t $IMAGE_TAG_NGINX . -f Dockerfile.nginx