Nice, glad to help!
As for the automatic restarting, the whole Docker container exits, if the main process (specified as CMD in the Dockerfile) exits (in this case the minecraft server) and Synology probably auto-restarts the containers on exit by default.
Late to the party, but I remembered this talk about maintaining a FreeBSD fork. If you want to get a more detailed description of what maintaining a customized OS entails, I encourage you to watch it: https://youtu.be/xddAX6L3iWc