[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
I'm still not clear why we're always running curl if the `SOURCE_URI` isn't git-based? Shouldn't we be doing this only if they've specified `CONTEXT_DIR`?