Initialize build args from stage base

Signed-off-by: Talon Bowler <[email protected]>

Author: daghack

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1034,6 +1034,7 @@ 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 {
@@ -2210,9 +2211,7 @@ func reportUnmatchedVariables(cmd instructions.Command, buildArgs []instructions
 		return
 	}
 	for _, buildArg := range buildArgs {
-		if buildArg.Value != nil {
-			delete(unmatched, buildArg.Key)
-		}
+		delete(unmatched, buildArg.Key)
 	}
 	if len(unmatched) == 0 {
 		return