How did you verify that your SSL certificate is working OK? What does https://www.sslshopper.com/ssl-checker.html report?
Also, for Central to work properly, it should be possible to access the host through its external IP from within each Docker container. In particular, if you can successfully curl your Central website over HTTPS on its public domain name, all web form features should work correctly. This is documented at https://docs.getodk.org/central-install-digital-ocean#changing-server-settings