If you need help with troubleshooting, start with posting your docker log
Also, it's not a good idea to test a reverse proxy right off the bat.
Set up the container first, check the logs to make sure the certs are retrieved correctly.
Then test to make sure you can get to the placeholder homepage.
Only then you should test the reverse proxy.
Step by step.
And here's a detailed guide that covers many scenarios: https://blog.linuxserver.io/2019/04/25/letsencrypt-nginx-starter-guide/