In which case it would make most sense to build a mock container on top which clearly exhibits the desired behavior of the underlying container and then test the behavior of that.
Maintainer of the tool here - I'm not sure I understand this idea. What would go in the mock container? How would you test against it? Would you mind elaborating a little more?
I'm assuming that in this case the base container would be a 'ruby' container or something. You'd build an example container atop it running a bit of example ruby code - verifying that it behaves properly. Those behavioral tests could be used to verify that the underlying container is configured properly.
I suppose that's sort of what you're doing in the example you linked to above.
I guess an "example". I generally agree with the parent poster. Testing for files does not sound useful. Test the functionality, not structurue. But not everyone's google.