Add missing properties to example compose files (#348888)

doc/install/docker.md

@@ -141,6 +141,8 @@ install, and upgrade your Docker-based GitLab installation:
 1. Create a `docker-compose.yml` file (or [download an example](https://gitlab.com/gitlab-org/omnibus-gitlab/raw/master/docker/docker-compose.yml)):
 
    ```yaml
+   version: '3.6'
+   services:
      web:
        image: 'gitlab/gitlab-ee:latest'
        restart: always
@@ -176,7 +178,9 @@ HTTP and SSH port. Notice how the `GITLAB_OMNIBUS_CONFIG` variables match the
 `ports` section:
 
 ```yaml
-web:
+version: '3.6'
+services:
+  web:
     image: 'gitlab/gitlab-ee:latest'
     restart: always
     hostname: 'gitlab.example.com'