WIP: Provide task to stage output in a format sufficient to build Docker images

This is a WIP to generate Docker images for applications which are packaged using sbt-native-packager.

It offers a docker:stage task which creates a directory sufficient to create images with
"docker build .", based on extending the layout from Universal.

It does not offer any deployment options, as that generally seems out of scope for this package.

I was wondering if you had any comments, or interest in merging functionality like this? Otherwise, it could also go into a separate plugin, and then might be able to include functionality to publish to Docker repositories.