v3.0: volumes.files becomes a dict, bug fixes

```
Traceback (most recent call last):
  File "/home/nbryskin/.local/bin/dominator", line 9, in <module>
    load_entry_point('dominator==3.0', 'console_scripts', 'dominator')()
  File "/home/nbryskin/src/dominator/dominator/actions/__init__.py", line 343, in main
    assert 1243 == 123
AssertionError
```