Docker mappings - incomplete?

I've just noticed that `docker:mappings` does not include the `DockerFile` that is produced. The `sbt-bundle` plugin uses `docker:mappings` as a general approach to including files when staging or producing a dist (it actually doesn't know anothing about the 'docker' config as this a setting to it). Should `docker:mappings` also include the `DockerFile` itself? I'm thinking so...

As an alternative the `File` result of `stage` could be used... only again it does not include the `DockerFile` as it points to `/files`. I'd prefer to use mappings though anyhow given that it retains the relative path.

What are your thoughts? `mappings` appears to be inclusive of the entire package with the other configurations doesn't it? /cc @jsuereth 
Will the internal path change? I mean, currently it is `/opt/docker/<app>` and for me
it looks like `/stage/opt/docker/<app>` now