Implement `RunningContainer.ip()`

Closes #3 (closed) - solution outlined in issue is not entirely followed. Unification and flag to alter behaviour is not included. Might not be needed.

This MR adds a single method to RunningContainer to retrieve the Ipv4Addr on the docker network assigned to this container. This will now allow us to contact containers started under dockertest from en environment where dockertest itself runs within a container (e.g., a CI runner).