Proxy to backend with default Next.js dev server

There is now an official feature for this in the config: Rewrites

Besides normal path rewrites, they can also proxy requests to another webserver

next.config.js:

module.exports = {
  async rewrites() {
    return [
      {
        source: '/api/:path*',
        destination: 'http://localhost:8000/:path*' // Proxy to Backend
      }
    ]
  }
}

See Next.js Docs Rewrites