Resolve CORS issue inside fastify

I have a contact form on my react APP that submits to a fastify API. The API sends email using Mailgun api. Works fine locally and at one point it worked fine from within chrome.

Now it does not work in any browser. Seems like a CORS issue.

The application has a dockerfile so I can invite you and you can run the container and test on your own.

To qualify for payment you need to ensure contact form works in all browsers and not create any security issues.