frontend/dockerfile/dockerfile2llb: errmsg: quote build target

The build target is not quoted and it makes it difficult for some
persons to see what the problem is.

By quoting it we emphasize that the target name is variable.

Signed-off-by: Frank Villaro-Dixon <[email protected]>

Author: Frankkkkk

frontend/dockerfile/dockerfile2llb/convert.go

@@ -317,7 +317,7 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 		var ok bool
 		target, ok = allDispatchStates.findStateByName(opt.Target)
 		if !ok {
-			return nil, errors.Errorf("target stage %s could not be found", opt.Target)
+			return nil, errors.Errorf("target stage %q could not be found", opt.Target)
 		}
 	}