Flatten on top

[Xe]

I'd like to be able to flatten the layers created by box as a layer on top of the base image in the from clause.

This would allow tenancy with multiple services using the the same base image without worker nodes needing to download the entire contents of the base image over and over again with each new release of the container. This would be entirely opt-in by means of explicitly telling flatten to do this.

EX:

from "centos:7"

# install some packages, etc
# set up a service to run

flatten :onTop

[Xe]

This is a dupe of #34