Why isn’t Express routing to static React build files w/o a URL slug?

The Setup

I’m building a React app that is statically served using Express.

• The React app is built with npm run build and the static files live in the build/ directory.

• Express statically serves these files with the following code (reference):

const path = require("path");
const express = require("express");
const app = express();
const port = 80;
const host = "::";

let root = path.join(__dirname, "build"); // Output of `npm run build`
app.use(express.static(root));  // Tell Express to serve static files from here

// The wildcard (*) lets all paths bypass Express 
// in favor of React handling client-side routing.
app.get("/*", (req, res) => {
  res.sendFile(path.join(root, "index.html"));
});

app.listen(port, host, () => {
  console.log(`Serving from root=${root}`);
  console.log(`Example app listening on host ${host} & port ${port}`);
});

I’m also containerizing the app and deploying it to EKS behind an Ingress (aka, an entrypoint).

• The Ingress URL is ingress.company.com
• My app has an address reserved at ingress.company.com/my-app. All users must be able to go here, so every URL should use this /my-app slug.

The Problem

When I point my browser to ingress.company.com/my-app, Express successfully returns build/index.html. However, the accompanying static JavaScript files aren’t being found resulting in a bare HTML file without life.

I’ve discovered the reason why:

• The static JavaScript file lives at ingress.company.com/my-app/js/main.hash.js
• Express is trying to reach files at ingress.company.com/js/main.hash.js (missing my-app/) and a 404 is returned.

How can I get my Static Express Server to include the URL slug my-app/ when serving for build files? Is there a way to set a Base URL?