title: service 'image' must be a mapping not a string
body: <p>I am following a tutorial on udemy and followed the exact steps the tutor did and wrote a docker compose file, but when I enter the command to build and run the app the following error occurs: In file '.\docker-compose.yml', service 'image' must be a mapping not a string.</p>
  
  <p>I tried changing the image to image: redis:alpine instead of image: redis as mentioned on docker docs but it still gives the same error, I even tried image: 'redis'.</p>
  
  <pre><code>version: '3'
  services:
   redis-server:
     image: redis
   node-app:
     build: .
   ports:
     - "4081:8081"
  </code></pre>