[DEVEXP-346] Allow docker type build from a repo with additional directory

This is the initial part of the work behind adding the additional directory to docker builds, especially this commit covers restructuring the `BuildInput` object splitting the docker related and STI related stuff into sub-objects. 
Still to do: 
- [x] docs
- [x] build image
Home dir is `/root` I thought of going the same way docker goes https://github.com/docker/docker/blob/master/api/client/commands.go#L117-L132.