Add named volumes

This MR introduces the possibility to attach named docker volumes to a Dockertest instance, and mount them into a container through a composition.

Dockertest will now create all named volumes supplied during object creation, and users can mount them into their container by passing the appropriate volume name and mount path to composition creation.

All volumes created by Dockertest are removed during cleanup.

I have not created dedicated structures for volumes in this MR, we can expand this when we add support for anonymous volumes.