builder: ONBUILD triggers were not accurately being executed in JSON cases

closes #8520

This re-invokes the parser for onbuild instructions. The method signatures for the dispatchers needed to change, so sorry about the extra noise.

/cc @crosbymichael @tiborvass @tianon

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)