update : Dockerfile

Hi @brennannovak,

the `Dockerfile` and the instructions in the wiki didn't work for me, so I refactored it based on the [vagrant-bootstrap.sh](https://github.com/pagekite/Mailpile/blob/37d608ef71dfa414e46e7c279b4584f0906cc35f/scripts/vagrant_bootstrap.sh).

I would also propose to run it as a deamon and add this info to [the respective wiki page](https://github.com/pagekite/Mailpile/wiki/Getting-started-for-developers) afterwards.

```
docker run -d -p 33411:33411 mailpile
```

Despite the changes look trivial, it took around 45mins time to figure it out, if you are tracking these kind of contributions.