Slightly improved Dockerfile

I made two minor improvements to the Dockerfile.

My reasons for doing this was to make it easier to use ngnix as a base image. In my case to make a series of packaged web-apps.

- Switched the nginx config back to default location /usr/local/nginx/conf/. Reason being it is easier to use it with volumes. /etc/nginx/ would work as well, but since it is built from source /usr/local is more appropriate.
- Added removal of source files. It may only be around 42MB but it does not belong in the production image.
As a RUN line by itself, this won't actually have any benefit at all thanks to the way Docker handles layering, but besides that, see also https://github.com/dotcloud/docker/blob/master/contrib/mkimage/debootstrap#L64-L87 for why the `apt-get clean` shouldn't be necessary either.