Self-Hosted Main

Docker provides some basic security guidelines here: https://docs.docker.com/engine/security/

But aside from specific containers and guidance, general network and system hardening guidelines would apply. You can look up plenty of server hardening guidelines via google. General principles such as least privilege, segmentation via VLANs and firewall rules, user ownership/privilege for accounts and services, will go a long way. Keep defense in depth in mind, so 1 control is none, 2 is one, and you can always find more ways to make something secure up to and including removal. The most secure thing, is a thing that doesn't exist.

There are also automated tools that can perform scans and 'audits' on your system, or your containers, to guide you on specifics you can adjust (such as lynis) and help lock you down in a more systematic way. These tools can be automated, report on a scheduled basis or one time use. One of those is your best bet for targeted and effective controls.