dockerfile: fix command count after new commands from ONBUILD

Fix progress lines like `[6/4] step`

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1134,6 +1134,7 @@ func initOnBuildTriggers(d *dispatchState, triggers []string, allDispatchStates
 		}
 	}
 	d.commands = append(commands, d.commands...)
+	d.cmdTotal += len(commands)
 
 	return hasNewDeps, nil
 }