builder: handle cases where onbuild is not uppercase.

closes #8675 

/cc @tiborvass 
Fixed

> On Oct 21, 2014, at 1:14 PM, Tibor Vass <notifications@github.com> wrote:
> 
> In integration-cli/docker_cli_build_test.go:
> 
> > @@ -15,6 +15,43 @@ import (
> >  	"github.com/docker/docker/pkg/archive"
> >  )
> >  
> > +func TestBuildOnBuildLowercase(t *testing.T) {
> > +	name := "testbuildonbuildlowercase"
> > +	name2 := "testbuildonbuildlowercase2"
> > +
> > +	defer deleteImages(name)
> > +	defer deleteImages(name2)
> deleteImages(name, name2)
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/8684/files#r19175762>.
>