All static files that you want to serve should be in the root directory of your repository. No need to use a separate `www` folder. `buildpack-nginx` will automatically download the buildpack, download NGINX, compile it, and install it. The next time you push your project, the buildpack will reuse the precompiled binaries.
You can override the nginx root via setting the `NGINX_ROOT` environment variable. This should be a relative path in your repository (for example `dokku config:set <app> NGINX_ROOT=_site` if you are deploying a Jekyll site).
You may completely override the built-in nginx config by placing an `app-nginx.conf.sigil` file in the root, modeled after our own [`conf/app-nginx.conf.sigil`](https://github.com/dokku/buildpack-nginx/blob/master/conf/app-nginx.conf.sigil). This will be used inside of the container, and not by the host Dokku instance. See the [sigil project](https://github.com/gliderlabs/sigil) for more information concerning the sigil format.