I am getting error while converting my next js project to docker

Ensure that you’re copying all files from the example. In this case you need to ensure you have added or customized next.config.js with this:

module.exports = {
  output: 'standalone'
}

You’ll notice the file is also defined in the examples: https://github.com/vercel/next.js/blob/canary/examples/with-docker/next.config.js