The error is not that it can’t find the certificate, it does find it. The problem is that it is a self signed certificate that isn’t trusted.
Not sure if you are using docker or run it directly on digital ocean. When running directly on digital ocean then I would expect it to trust its own generated certificate. When running in docker you probably have to do some extra work to trust it.
An other solution is to ignore the untrusted certificate, add rejectUnauthorized: false
to the ssl option.