Docker and NextAuth with Google OAuth are not working

[datteroandrea]

So I have an authentication system that uses NextAuth and Google OAuth and when using it in development (running on http://localhost:3000) the authentication succedes however at the moment for production purposes I had to deploy the application into a Docker container and using Nginx the server is now reachable via http://localhost however when i try to autenticate now I get the following error:

[next-auth][error][CLIENT_FETCH_ERROR] 
https://next-auth.js.org/errors#client_fetch_error fetch failed {
error: {
   message: 'fetch failed',
   stack: 'TypeError: fetch failed\n' +
   '    at Object.processResponse (node:internal/deps/undici/undici:6406:34)\n' +
   '    at node:internal/deps/undici/undici:6727:42\n' +
   '    at node:internal/process/task_queues:140:7\n' +
   '    at AsyncResource.runInAsyncScope (node:async_hooks:203:9)\n' +
   '    at AsyncResource.runMicrotask (node:internal/process/task_queues:137:8)\n' +
   '    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)',
      name: 'TypeError'
   },
   url: 'http://localhost/api/auth/session',
   message: 'fetch failed'
}

Looking at the error it appears I should not have correctly set NEXTAUTH_URL as seen here from the previous error log.
However if you take a look at my .env file you will see the following:

NEXTAUTH_URL=http://localhost/

As you can see I have correctly set the NEXTAUTH_URL and if I try to change it to http://localhost:3000/ then after signing in I will be redirected to http://localhost:3000/api/auth/signin?csrf=true which does not show anything because the server is now accessible only via http://localhost.

As you may imagine I may have made a mistake in the Google Developer Console, however I can guarantee that I have set both urls in the Google OAuth to http://localhost.

How can I solve this problem?