[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
Go was already proposed in #143, I'd prefer to move all the talks regarding builder changes there, if you don't mind. It was already talked in couple of places mentioned in this proposal.