Syntax error in example docker compose

Problem to solve

A few docker-compose.yml examples are provided the Install GitLab using Docker Compose documentation. When following the instructions, I copied one of the examples and wrote it to a docker-compose.yml file. However, using docker-compose commands with this file failed:

$ docker-compose convert
(root) Additional property web is not allowed

Further details

$ cat <<EOF >docker-compose.yml

web:
  image: 'gitlab/gitlab-ee:latest'
  restart: always
  hostname: 'gitl ab.example.com'
  environment:
    GITLAB_OMNIBUS_CONFIG: |
      external_url 'https://gitlab.example.com'
      # Add any other gitlab.rb configuration here, each on its own line
  ports:
    - '80:80'
    - '443:443'
    - '22:22'
  volumes:
    - '$GITLAB_HOME/config:/etc/gitlab'
    - '$GITLAB_HOME/logs:/var/log/gitlab'
    - '$GITLAB_HOME/data:/var/opt/gitlab'
  shm_size: '256m'

EOF

$ docker-compose convert
(root) Additional property web is not allowed

Proposal

Include version: '3.6', and nest web under services:

version: '3.6'
services:
  web:
    image: 'gitlab/gitlab-ee:latest'
    restart: always
    hostname: 'gitlab.example.com'
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        external_url 'https://gitlab.example.com'
        # Add any other gitlab.rb configuration here, each on its own line
    ports:
      - '80:80'
      - '443:443'
      - '22:22'
    volumes:
      - '$GITLAB_HOME/config:/etc/gitlab'
      - '$GITLAB_HOME/logs:/var/log/gitlab'
      - '$GITLAB_HOME/data:/var/opt/gitlab'
    shm_size: '256m'