[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
Knowing that we had CVEs in the past on this in Openshift w.r.t. Url escaping I'd say we should learn from it.

Eventually we need a better way to firewall the operations the container can execute against docker (must run in this security context etc) and this problem mostly goes away because the builder can't do anything particularly dangerous.


> On Oct 10, 2014, at 2:47 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 would have said defer, but if you're afraid of the $envVars, it might make sense to do the switch now.
> 
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