vendor: add buildkit

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

vendor/github.com/moby/buildkit/session/context.go

@@ -0,0 +1,22 @@
+package session
+
+import "context"
+
+type contextKeyT string
+
+var contextKey = contextKeyT("buildkit/session-id")
+
+func NewContext(ctx context.Context, id string) context.Context {
+	if id != "" {
+		return context.WithValue(ctx, contextKey, id)
+	}
+	return ctx
+}
+
+func FromContext(ctx context.Context) string {
+	v := ctx.Value(contextKey)
+	if v == nil {
+		return ""
+	}
+	return v.(string)
+}