Proxy express backend from Create React App in dev and prod - reactjs

Question

I'm new to full stacking and have a Create React App hitting a single /api/verify endpoint in an Express backend.
In dev, I proxy the backend in my frontend package.json with "proxy": "localhost:8081"
DevOps needs to deploying this to a GCP nginx env and I need to make the proxy path relative so we don't have to hard code the domain in the proxy.
I have create a .env.development and .env.production but I'm not sure how to use them correctly so a local yarn start proxies localhost:8081 and a production build proxies the real domain.
I have a typical, simple Express server.js but not sure how to tie it all together.
const cors = require('cors');
const express = require('express');
const bodyParser = require('body-parser');
require('dotenv').config();
const app = express();
var corsOptions = {
origin: 'http://localhost:8081',
};
app.use(cors(corsOptions));
app.use(express.json());
app.use(bodyParser.json());
app.disable('x-powered-by');
// parse requests of content-type - application/x-www-form-urlencoded
app.use(bodyParser.urlencoded({ extended: true }));
// simple route
app.get('/', (req, res) => {
res.json({ message: 'Welcome to bezkoder application.' });
});
require('./routes/gallery.routes')(app);
const PORT = process.env.PORT || 8081;
app.listen(PORT, () => {
console.log(`Server is running on port ${PORT}.`);
});
And my single route:
const { authJwt } = require('../middleware');
const galleryController = require('../controllers/gallery.controller.js');
module.exports = function (app) {
app.use(function (req, res, next) {
res.header(
'Access-Control-Allow-Headers',
'x-access-token, Origin, Content-Type, Accept',
);
next();
});
app.get(
'/api/verify',
[authJwt.verifyToken],
galleryController.fanPassGallery,
);
};