Except that multi-stage builds are deceptively not grabbing or blending layers - it's simply letting you copy files from a layer. And that is a big miss. Being

Except that multi-stage builds are deceptively not grabbing or blending layers - it's simply letting you copy files from a layer. And that is a big miss. Being able to describe "now append this layer and all, but only, its file changes from this previous layer" would be pretty epic.