fix(container): update security settings of favonia/cloudflare-ddns

[favonia]

Thanks for using my DDNS updater. Since version 1.13.0 (released on 16 July), the updater has stopped dropping superuser privileges by itself, instead relying on Docker's built-in mechanism to drop those privileges. The new way is safer, cleaner, and more reliable; but it requires an update to the configuration. In particular, the environment variables PUID=uid and PGID=gid should be replaced by user: "uid:gid" or --user uid:gid. I am on a mission to eliminate the old template from the internet. Please help me promote security best practices!

For more information about this design change, please read the CHANGELOG. If copyright ever matters, this PR itself is licensed under CC0, which should allow you to do whatever you want. Thank you again for your interest in the updater.

PS: I know you are using an older version of the updater, but the template works even for older ones. (An upgrade is recommended, though.)
PPS: I do not have a VyOS to test the script. Please let me know whether it works or not. BTW, it’s sad that VyOS does not seem to provide many other useful protections such as “dropping all Linux capabilities” or “making the filesystem read-only”.