Implement dockerfile output

This Pull Request will well-work after serverspec/specinfra/pull/260.

You said that like this Pull Request on p45 of [your slide](https://speakerdeck.com/ryotarai/itamae-infra-as-code-xian-zhuang-que-ren-hui) and I need it too, so I've implemented.

Would you say that this commits have gone in the right direction? I think Dockerfile output can be implemented in diverse ways.