Failed writing file permission denied

When trying to parse a consul template within a docker container I am getting the following error. I searched through the backlog of issues and couldn't find a solution. I assume it is trying to write a temp file - hence the 865492878

I'm passing the following in as template

```
--template /config/config.ctmpl:/config/config.json
```

And getting the following error

```
2018/05/04 16:15:26.143342 [ERR] (cli) error rendering "/config/config.ctmpl" => "/config/config.json": failed writing file: open /config/865492878: permission denied

```

Any help would be greatly appreciated