Simply run
rackup -q -p 8080
As you can see from the config.ru, it will forward all requests according to this table:
| Method | Route | Target |
|---|---|---|
| GET | /api/* | http://jsonplaceholder.typicode.com/* |
| GET | /api/customers/* | http://jsonplaceholder.typicode.com/users/* |
| POST | * | HTTP Error 403 |
| PUT | * | HTTP Error 403 |
| PATCH | * | HTTP Error 403 |