Add image domain as an env var

[Andriy-Kulak]

First off, thanks for the amazing repo! Not a major issue, but one piece of quick feedback is to make image domain url as part of the env vars within next.config.js. I had to change it to get your repo to work locally and in prod with my blob setup:

Reference: https://github.com/Nutlope/qrGPT/blob/main/next.config.js

You can change to something like below & add a note to .example.env

/** @type {import('next').NextConfig} */
const nextConfig = {
  images: {
    domains: ['pbxt.replicate.delivery', process.env.IMG_DOMAIN],
  },
};

module.exports = nextConfig;