Invalid stackfile. See the stack YAML reference for more details

It’s proving impossible to add a simple stackfile without throwing a yaml error. I’ve tried tabs, spaces, everything, but can’t seem to resolve it. This example is with spaces.

website:
  image: registry.gitlab.com/username/docker-website:latest
  ports:
    - "80:80"
    - "443:443"
  volumes:
    - storage:/var/www

drupal:
  image: registry.gitlab.com/username/docker-drupal:latest
  volumes:
    - storage:/var/www

What’s wrong with this stackfile?

‘storage’ is not defined?