docs/example/docker: Add COOKIE_PATH example to compose (#437)

docs/examples/docker-compose.example.yml

@@ -21,6 +21,8 @@ services:
             API_URL: "https://co.wuk.sh/"
             # replace eu-nl with your instance's distinctive name
             API_NAME: "eu-nl"
+            # if you want to use cookies when fetching data from services, uncomment the next line and the lines under volume
+            # COOKIE_PATH: "/cookies.json"
             # see docs/run-an-instance.md for more information
         labels:
             - com.centurylinklabs.watchtower.scope=cobalt