Allow apps to run without root

Per https://github.com/progrium/dokku/issues/89, this allows apps to run as a non-root user. It would require some small changes on the dokku side to work, but I wanted to put this here to get reviewed first.

Here was my process for testing it:

```
make
cd $my_ruby_app
git archive HEAD | $buildstep_dir/buildstep appname
docker run -u www-data -d -p 5000 -e PORT=5000 appname /start web
```