Docker support could allow building Dockerfiles or otherwise run extra commands to initialize a container

As I just briefly discussed with @chreekat and @mgsloan on IRC, I have a situation where I need a derived Docker image for my project since I need some extra system dependencies installed for both building and running tests.

It seems the main benefit of using stack's docker support (instead of just rolling my own docker image and running stack inside of it) is that it Just Works without any developer setup. But as soon as I need a custom image like this, that benefit goes away, since my developers need to explicitly build the image now.

So it would be nice if I could specify a Dockerfile in the `docker` section of my stack.yaml, or perhaps just a small script of commands to run inside the docker environment before building (assuming the result of that script would then be cached in the docker image so it wouldn't need rerun every time I call `stack build`).

This is a pretty minor issue in the grand scheme of things, but it seems something that fits in with the intent of stack's docker support.