`--target-dir` results in `failed to create directory /target/release`

Cross is running docker with `--user`, and when you use `--target-dir` it adds a volume mount from the directory to `/target`, like so:

```bash
docker run <...> -v mytargetdir:/target:Z <...>
```

The `/target` directory will be owned by root, and cross will fail to create anything there.
