The "sync" command in backup-loop.sh

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.

Start by reading backup-loop.sh and tracing the retry invocation following save-all. Verify the command target inside the mc-backup container and compare it with the surrounding rcon-cli calls. Done means Minecraft receives the sync command rather than the container's Linux sync command.

Written by the indexing model from the issue text.