Where does 'stack build PKG' install 'PKG'; how do I isolate the install of 'PKG'?

Question

I'm confused about where stack build PKG places the packages it installs. I had thought that, for example

mkdir foo
cd foo
stack build PKG_IN_HACKAGE_NOT_IN_RESOLVER

would create foo/.stack-work and put the specified package there (as if to had been specified as an "extra dependency"); but I seen on .stack-work and the package is available globally on my system.

If what I want is an entirely isolated (temporary) Haskell configuration in which to install and use PKG, how do I do that?