Custom 404 with Gatsby and Now

now.json

{
  "version": 2,
  "name": "xxx",
  "builds": [
    {
      "src": "package.json",
      "use": "@now/static-build",
      "config": { "distDir": "public" }
    }
  ],
  "routes": [
    { "handle": "filesystem" },
    { "src": "/(.*)", "dest": "/404.html", "status": 404 }
  ]
}