Updated docker-py to 0.6.0 to fix issues with directories and symlinks

By updating docker-py to version 0.6.0 an issue is resolved where symlinks and empty directories were not properly passed to docker when building an image. See https://github.com/docker/fig/issues/651. This issue resulted in images that were different from images built with the default docker client.

I successfully ran all the tests. 

Output:

    Ran 156 tests in 206.877s

    OK