[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
Agree


> On Oct 10, 2014, at 2:25 PM, Cesar Wong <notifications@github.com> wrote:
> 
> In images/builder/docker/docker-builder/build.sh:
> 
> >      exit 1
> >    fi
> >  fi
> >  
> > -docker build --rm -t $TAG $DOCKER_CONTEXT_URL
> It seems we would lose some function if we now always download the source ourselves, instead of letting docker deal with it when the context directory is not specified. See https://github.com/docker/docker/blob/master/api/client/commands.go#L140-L157
> 
> —
> Reply to this email directly or view it on GitHub.