Document container user setting to match /data ownership

> But this setting is difficult to understand for me. I wish this thing would be documented.

Good point. I'll find a place to document and probably update the example to include it.

_Originally posted by @itzg in https://github.com/itzg/docker-minecraft-server/issues/3525#issuecomment-3032805435_

Start by locating the documentation and example for the container user setting and /data ownership. Document the setting and update the example to include it; done when users can understand how to configure it from the project documentation.

Written by the indexing model from the issue text.