How to setup rsync

As far as I know rsync should only do one full backup and then have only backups of the changes.
But why do I get only full backups all the time? Did I configure something wrong?

compose.yml
```
services:

mc:
container_name: mc-vanilla-plus
image: itzg/minecraft-server
stdin_open: true
ports:
- "25565:25565"
- "25575:25575"
volumes:
- ./data:/data
depends_on:
restore_backup:
condition: service_completed_successfully
environment:
EULA: true
MEMORY: "6G"
MOD_PLATFORM: "MODRINTH"
MODRINTH_MODPACK: "cutika-vanilla+-server.mrpack"
RCON_PASSWORD: "####"
restart: unless-stopped

restore_backup:
container_name: restore-vanilla-plus
image: itzg/mc-backup
restart: "no"
entrypoint: restore-rsync-backup
volumes:
- ./data:/data
- ./backups:/backups:ro

backups:
container_name: backups-vanilla-plus
image: itzg/mc-backup
depends_on:
mc:
condition: service_healthy
environment:
BACKUP_INTERVAL: "2h"
BACKUP_METHOD: "rsync"
RCON_HOST: mc
INITIAL_DELAY: 0
volumes:
- ./data:/data:ro
- ./backups:/backups
```

Start with the supplied compose.yml, especially the backups service and its BACKUP_METHOD setting, then inspect the restore-rsync-backup entrypoint used by restore_backup. Compare how successive runs populate /backups and determine whether the configuration or rsync behavior explains the full backups; done means the cause and required configuration are clearly documented.

Written by the indexing model from the issue text.