- Closes apple#508.

## Type of Change
- [ ] Bug fix
- [x] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
Adds a `container volume prune` command that removes volumes with no
container references and reports the amount of disk space reclaimed.
This helps users clean up unused volumes and easily reclaim disk space.

Also updates the `volume delete` documentation to clarify and highlight
how the `--all` flag works.

## Testing
- [x] Tested locally
- [x] Added/updated tests
- [x] Added/updated docs