[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



> On Oct 10, 2014, at 1:22 PM, Andy Goldstein <notifications@github.com> wrote:
> 
> In images/builder/docker/docker-builder/build.sh:
> 
> >  fi
> >  
> > -if [[ $DOCKER_CONTEXT_URL != "git://"* ]] && [[ $DOCKER_CONTEXT_URL != "git@"* ]]; then
> > -  URL=$DOCKER_CONTEXT_URL
> > +if [[ $SOURCE_URI != "git://"* ]] && [[ $SOURCE_URI != "git@"* ]]; then
> I'm starting to wonder if this needs to be written in Go instead of bash...
> 
> @smarterclayton
> 
Yes, is this worth doing now or defer to next change?

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