[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
Don't do this - none of our object types should have methods - they are purely value objects.  Why do you need this?  Just switch on `case input.STIInput != nil` in your code