Builder: extract the execution of a single build step into buildFile.BuildStep

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
builder: clearly display ONBUILD triggers during a build.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #3254 from shykes/onbuild

New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build