Use host if there is no alias

[michivi]

This is an attempt to work around #42. It does by making sure Ryuk is accessed using the Docker host.

This PR includes:

• (Breaking change) containerAlias can now return Nothing if there is no associated network or alias. It happens for containers in the default bridged network, like Ryuk.
• containerAddress will attempt to use the default Docker host address if it cannot find the container alias. It would be better to leave that as a configuration parameter in the future though.