Add docker-compose example about rake secret task

.env.production.sample

@@ -12,7 +12,7 @@ LOCAL_DOMAIN=example.com
 LOCAL_HTTPS=true
 
 # Application secrets
-# Generate each with the `rake secret` task
+# Generate each with the `rake secret` task (`docker-compose run --rm web rake secret` if you use docker compose)
 PAPERCLIP_SECRET=
 SECRET_KEY_BASE=