version: '3.1'
services:
  #Nginx Service
   webserver:
     image: nginx:alpine
     container_name: webserver
     restart: unless-stopped
     ports:
       - "80:80"
       - "443:443"

git clone https://github.com/collabnix/dockerlabs
cd intermediate/workshop/compose/lab/3/config/
cat docker-compose.yml

$ docker-compose config
services:
  webserver:
    container_name: webserver
    image: nginx:alpine
    ports:
    - 80:80/tcp
    - 443:443/tcp
    restart: unless-stopped
version: '3.1'

$docker-compose config

ERROR: The Compose file './docker-compose.yml' is invalid because:
Invalid top-level property "service". Valid top-level sections for this Compose file are: services, secrets, version, networks, volumes, and extensions starting with "x-".

You might be seeing this error because you're using the wrong Compose file version. Either specify a supported version (e.g "2.2" or "3.3") and place your service definitions under the `services` key, or omit the `version` key and place your service definitions at the root of the file to use version 1.
For more on the Compose file format versions, see https://docs.docker.com/compose/compose-file/