Docker tmpnb

This is the base [Docker](https://www.docker.com/) image that we can use with [tmpnb](https://github.com/jupyter/tmpnb) to provision each user's environment.

@richvn, this `Dockerfile` outlines how to build an environment that can be deployed to any Linux host. The way I build this is running:

```
docker build -t jupyter/nature-demo .
```

Then to run it directly, I use

```
docker run -it -p 8888:8888 jupyter/nature-demo
```

This makes the nature demo image appear on e.g. http://127.0.0.1:8888. When used in the tmpnb environment, this all happens in the background.