Changes:
- Upgrade Kerko to version
[0.3](https://github.com/whiskyechobravo/kerko/blob/master/CHANGELOG.md#03-2019-07-29).
- Provide a Docker container with KerkoApp
([6](https://github.com/whiskyechobravo/kerkoapp/pull/6),
[7](https://github.com/whiskyechobravo/kerkoapp/pull/7)). Thanks
[Emiliano Heyns](https://github.com/retorquere).
- Read more Kerko configuration variables from the environment.
- Improve documentation.