Getting HTTPS on a Synology NAS + Gitlab container is a bit tricky. Using self-assigned OpenSSL certificates is great, but it can only provide SSL certificates that inevitably will be flagged as untrusted by the browser due to the common name being unrecognized/not associated with a trusted SSL provider:
The downside will be that every user that remotely accesses your NAS will be greeted with the above message unless they manually add the certificate to their browser's approved SSL provider list. Instead, here's a work-around to enable HTTPS for both your Synology NAS and a Gitlab container using just one Let's Encrypt certification.
For more information regarding the docker-gitlab installation and set up: Synology Docker