Document how to build a package like stackage-curator does

The CURATORS.md document explains how building a new Stackage snapshot works. But this relies on a build.sh script, which as far as I can tell a) requires special access rights to run successfully, b) supports snapshot targets, but not package targets.

My problem: I'd like to test whether stackage-curator will succeed in the Docker build image, using the parameters inferred by build-constraints.yaml.

This is particularly relevant for #2003. There, we have a package that depends on system dependencies, and depends on `--extra-lib-dirs`, `--extra-include-dirs` to be set properly to build. I can reproduce the Docker image local given the in-repo Dockerfile and test the build manually using Stack and manually set build arguments on the command-line, but then curators using `./build.sh` still get a different result from my manual testing.

What I'd like: have a documented way to say, "build package `foo` (and only `foo` and its dependencies) in the build image like build.sh would do", i.e. using the same build arguments as those that would be inferred from `build-constraints.yaml`.

This is particularly interesting to have given that Travis CI doesn't test whether a package actually builds. Only that a build plan still exists.