docs: clarify docker-compose usage

Author: FinalAngel

docker-compose.yml

@@ -1,5 +1,6 @@
-version: "3"
-
+# The docker-compose.yml file is __only__ used for local development.
+# This means that changes to this file will not affect cloud deployments in any way.
+# Read more at https://docs.divio.com/reference/docker-docker-compose/
 services:
   web:
     build: .
@@ -12,6 +13,8 @@ services:
       DATABASE_URL: postgres://postgres@database_default:5432/db
     links:
       - "database_default"
+    # The following command is used to start the local development server.
+    # Removing this line will mimic the deployment behaviour on Divio Cloud.
     command: python manage.py runserver 0.0.0.0:80
 
   database_default: