Separate runtime from build dependencies and remove build dependencies

Hi! How about shrinking the image size on this one, as we did with https://github.com/docker-library/redis , removing build dependency packages and leaving only runtime dependency packages?

I've already forked at https://github.com/vovimayhem/nginx-docker/tree/feature/shrink-image-size , and shrunk the image from 499.1 MB to 213.8 MB... but I think the Dockerfile is kinda messy...

What do you think?