[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
Well, that's kind of happy guess ;) I needed some reasonable value to make sure the net lag isn't a problem here, thus I've chosen 16 ;)