Using debian:jessie instead of Ubuntu

[Debian is a smaller image](http://blog.javabien.net/2014/06/18/size-of-docker-images-which-linux-is-smaller/) and it works with it.

I've also added `0.0.0.0` as the hostname, since docker containers have their own hostname and IP, and I don't think anyone would like to mess with his `/etc/hosts` file every time he launches a new `fake-s3` instance.

and also, it may be the best to add `-p 4569:4569` to the `README.md` file