Initialize build args from stage base

Signed-off-by: Talon Bowler <talon.bowler@docker.com>
(cherry picked from commit c7c2f0e)

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1035,6 +1035,7 @@ func (ds *dispatchState) init() {
 	// the paths we use back to the base image.
 	ds.paths = ds.base.paths
 	ds.workdirSet = ds.base.workdirSet
+	ds.buildArgs = append(ds.buildArgs, ds.base.buildArgs...)
 }
 
 type dispatchStates struct {