Could not change group id to that of run_as user 'nobody': Invalid argument

### What version of shadowsocks-libev are you using?

Edge

### What did you do?

```dockerfile
FROM shadowsocks/shadowsocks-libev:edge

USER nobody
```

```bash
docker run -p 8388:8388 --user 1000:1000 shadowsocks/shadowsocks-libev:latest
```
