title: docker-compose build image or build context
body: <p>I would like build custom Dockerfile, but i've this message : 
  <code>
  --&gt; Starting build app
  ERROR: The Compose file is invalid because:
  Service app has neither an image nor a build context specified. At least one must be provided.
  </code></p>
  
  <p>I don't understand why the name is <code>app</code>. You can see my docker-compose.yml file : </p>
  
  <pre><code>version: "3"
  
  services:
    webapp-react:
      build:
        context: ./docker/nodejs
        dockerfile: Dockerfile
      image: webapp/react
      container_name: webapp_react
      user: ${UID}:${GID}
      environment:
        - NODE_ENV=build
  
    webapp-nginx:
      build:
        context: ./docker/nginx
        dockerfile: Dockerfile
      image: webapp/nginx
      container_name: webapp_nginx
      environment:
        - VIRTUAL_HOST=${SITE_URL}
        - VIRTUAL_PORT=8080
        - VIRTUAL_NETWORK=nginx-proxy
        - LETSENCRYPT_HOST=${SITE_URL}
        - LETSENCRYPT_EMAIL=${MAILER_DEFAUT_SENDER_ADDRESS}
      ports:
        - "8181:80"
      volumes:
        - ./app/build:/www-data/
  
  networks:
    default:
      external:
        name: nginx-proxy
  </code></pre>
  
  <p>Anyone know where did I make an error ?</p>
  
  <p>Thank you !</p>