no clue how to do this

.gitlab-ci.yml

+stages:
+  - build
+
+image: docker:20.10.8
+
+variables:
+  DOCKER_TLS_CERTDIR: "/certs"
+  IMAGE_TAG_APP: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG
+  IMAGE_TAG_NGINX: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-nginx
+
+services:
+  - docker:20.10.8-dind
+
+before_script:
+  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+
+build_app:
+  stage: build
+  script:
+    - docker pull $CI_REGISTRY_IMAGE:test || true
+    - docker build --cache-from $CI_REGISTRY_IMAGE:test -t $IMAGE_TAG_APP .
+    - docker push $IMAGE_TAG_APP
+
+build_nginx:
+  stage: build
+  when: manual
+  script:
+    - docker pull $CI_REGISTRY_IMAGE:test-nginx || true
+    - docker build --cache-from $CI_REGISTRY_IMAGE:test-nginx -t $IMAGE_TAG_NGINX . -f Dockerfile.nginx
+    - docker push $IMAGE_TAG_NGINX