Stick the args to CMD in a list. Fixes #3.

Well, I finally figured out what was going on with slow shell access. It was all about how the Dockerfile was laid out.

```
CMD /notebook.sh
```
is very different from
```
CMD ["/notebook.sh"]
```

From the [section on `CMD` in the Docker docs](https://docs.docker.com/reference/builder/#cmd):

> If you use the shell form of the `CMD`, then the `<command>` will execute in `/bin/sh -c`

Switching over made the timings sane.