failed to resolve output format "print" from site config

Following the getting started instructions, I get the following error:

```
$ docker-compose up
Creating network "mydocs_default" with the default driver
Creating mydocs_site_1 ... done
Attaching to mydocs_site_1
site_1  | Error: from config: failed to resolve output format "print" from site config
mydocs_site_1 exited with code 255
```

