rootlesskit-docker-proxy should report detailed error via FD 3

```console
$ docker --context=rootless run -it -p 80:80 --rm alpine
docker: Error response from daemon: driver failed programming external connectivity on endpoint clever_euclid (77c992ce4056362ee89727f4d2d96c349bbb9013b14f4f2012412f2fbc94718e): Error starting userland proxy:.
```

rootlesskit-docker-proxy should report detailed error via FD 3.