[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
This will create the temp dir in /tmp. Is there any chance of running out of space? Do we really need to use `mktemp` here - couldn't we just let git clone into some name we choose in the current dir?