vendor: add buildkit

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/buildkit/client/buildid/metadata.go

@@ -0,0 +1,29 @@
+package buildid
+
+import (
+	"context"
+
+	"google.golang.org/grpc/metadata"
+)
+
+var metadataKey = "buildkit-controlapi-buildid"
+
+func AppendToOutgoingContext(ctx context.Context, id string) context.Context {
+	if id != "" {
+		return metadata.AppendToOutgoingContext(ctx, metadataKey, id)
+	}
+	return ctx
+}
+
+func FromIncomingContext(ctx context.Context) string {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		return ""
+	}
+
+	if ids := md.Get(metadataKey); len(ids) == 1 {
+		return ids[0]
+	}
+
+	return ""
+}