Piping data into systemd-docker doesn't forward it to docker

Example of where this is done:
https://github.com/pablocouto/coreos-sshguard

```
journalctl --no-pager -f -n0 -q -t sshd | /usr/bin/docker run -i --name=%n -v /dev/log:/dev/log --net=host --cap-add=NET_ADMIN --cap-add=NET_RAW pablocouto/coreos-sshguard
```