How to add SSL to Sync endpoints server with Lets encrypt?

1. What is the problem? Be very detailed.
Is there any documentation or basic guidelines for adding SSL to an ODK-X sync endpoints server using Lets Encrypt? I’d like to encrypt and I have a domain name, but I don’t have a wildcard SSL.

2. What app or server are you using and on what device and operating system? Include version numbers.
ODK-X sync endpoints, hosted by Digital Ocean, uses Docker, Ubuntu 18.04.1 x64

3. What you have you tried to fix the problem?

4. What steps can we take to reproduce the problem?

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.

There is sparse documentation under the HTTPS section.

See: https://docs.opendatakit.org/odk-x/sync-endpoint/#https