dockerTools.buildLayeredImage: pass a list of closures to mkManyPureLayers so...

dockerTools.buildLayeredImage: pass a list of closures to mkManyPureLayers so it can exclude the top-most level

Before, every docker image had three extra layers:

1. A `closure` layer which is an internal implementation detail of
   calculating the closure of the container
2. a `name-config.json` layer which is the images' run-time
   configuration, and has no business being *in* the image as a layer.
3. a "bulk-layers" layer which is again and implementation detail
   around collecting the image's closure.

None of these layers need to be in the final product.