● nginxの設定ファイル

      location /app/ {
          proxy_pass http://localhost:3000/;
      }

● next.config.js

assetPrefix を追加します

const SUB_DIRECTORY = "/app";

const isProduction = process.env.NODE_ENV === "production";

/** @type {import('next').NextConfig} */
const nextConfig = {
  assetPrefix: isProduction ? SUB_DIRECTORY : "/" ,
  reactStrictMode: true,
  swcMinify: true,
};

module.exports = nextConfig;