add docker support to EventLoop batch drivers

That's the very first version, allowing to run inside docker in LocalDriver, and in a newly created KubernetesDriver (which also only works locally for now).

This is probably not that useful as it stands for now, but there are more updates to come in separate merge requests:

• support docker in other batch drivers besides those two
• support docker in the grid driver
• allow docker running without a shared file system
• make the kubernetes running more flexible

Though all of that will also be interleaved with some refactoring of the EventLoop implementation as well, so I'm not sure in what order this will come.

cc @lheinric