docker: failed to create lightpanda data dir: error.FileNotFound

In the docker build, we fail to create the lightpanda data dir.

```
$ docker build -t lightpanda-docker .
```

```
$ docker run --privileged lightpanda-docker
warning(app): failed to create lightpanda data dir: error.FileNotFound
info(telemetry): telemetry enabled
info(server): accepting new conn...
warning(telemetry): server error status: http.Status.bad_request
```