Race Condition in Retrieval of Container PID

I was/am struggling with getting Docker instances to boot up properly. The exception can be traced down to `_find_container_pid` in the  driver. It can't find that `tasks` file.

From the logs and some debugging I could see though that the instance was created and the file was actually there and contained the PID.

It seems there is a race condition or a small startup delay. Adding the sleep solved the problem for testing purposes. Unfortunately I don't really know how to solve that problem idiomatically. So, consider this a heads up that there is something going on here...

Impressive how simple the integration is. Docker is quite cool!