Copy and run prepare before builder to better leverage docker caching

ATM, every time I do a change on `./stack/builder` file, and run `make build`, it downloads all the packages and it takes forever!
`/prepare` is a slow operation, so we need to make sure we only run it, when strictly necessary, so we cannot copy  the entire `/stack` folder, because if any change happens in it, for instance a change on `builder`, it would invalidate the cache from the point on, and run prepare again.

review @progrium