The following line in backup-loop.sh is missing the "rcon-cli" portion of the command:
retry ${RCON_RETRIES} ${RCON_RETRY_INTERVAL} sync

This results in the command being executed in the mc-backup container instead, calling the Linux "sync" command. There is no need for this command to run inside the mc-backup container and it appears to have been intended to be the sync command in Minecraft server, which should follow the save-all command.

Suggested fix is to simply add the "rcon-cli" command to this line. Will submit a PR; opening this issue for tracking purposes.