add docker image support

Instead of installing this project locally, I created a `Dockerfile` which builds an image that has everything ready to use. This is helpful to me because then I don't have to worry about installing and dependencies on my local system, I only need Docker installed correctly.

All you have to do is create a `assetgraph-builder` image on the [Docker Hub](https://registry.hub.docker.com/) and turn on [automated builds](https://docs.docker.com/docker-hub/builds/)

Let me know if you have any questions.