change builder API to return imageID
change builder API to return imageID

this allows more flexibility running CLI builder, especially we don't
have anymore to parse build output to get build status/imageID and can
pass the console FDs to buildkit

Signed-off-by: Nicolas De Loof <27d8e526c390473e9c51da80626c6b0539b42f4f@gmail.com>

thaJeztah/compose,thaJeztah/compose,vdemeester/compose,vdemeester/compose