Update gomod and vendor

vendor/github.com/moby/buildkit/util/winlayers/context.go

@@ -0,0 +1,19 @@
+package winlayers
+
+import "context"
+
+type contextKeyT string
+
+var contextKey = contextKeyT("buildkit/winlayers-on")
+
+func UseWindowsLayerMode(ctx context.Context) context.Context {
+	return context.WithValue(ctx, contextKey, true)
+}
+
+func hasWindowsLayerMode(ctx context.Context) bool {
+	v := ctx.Value(contextKey)
+	if v == nil {
+		return false
+	}
+	return true
+}