Adds a Dockerfile

This Dockerfile can be used for building a Docker image of Webogram that
launches a container with Node listening on port 8000.

This image uses the official nginx image as its base.

----

If you accept this PR it'd also be awesome if you could set up an [automated build](http://docs.docker.com/docker-hub/builds/) on [Docker Hub](https://hub.docker.com/) and maybe link it from the `README`. It's always very nice to have builds from an official repository and doesn't take very long to set up!