Add automated daily Gitea backup

Alpine sidecar container runs gitea dump via docker exec at 2 AM daily,
writing backups to /data/backup/gitea/ with 7-day retention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -15,9 +15,33 @@ services:
       - gitea_data:/data
       - /etc/timezone:/etc/timezone:ro
       - /etc/localtime:/etc/localtime:ro
+      - /data/backup/gitea:/tmp/backup
     ports:
       - "3008:3000"
       - "2222:22"
 
+  gitea-backup:
+    image: alpine:3.21
+    container_name: gitea-backup
+    depends_on:
+      - gitea
+    restart: unless-stopped
+    environment:
+      - BACKUP_RETAIN_DAYS=7
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+      - /data/backup/gitea:/backup
+      - ./backup.sh:/usr/local/bin/backup.sh:ro
+      - /etc/timezone:/etc/timezone:ro
+      - /etc/localtime:/etc/localtime:ro
+    entrypoint: /bin/sh
+    command:
+      - -c
+      - |
+        apk add --no-cache docker-cli
+        echo "0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1" > /etc/crontabs/root
+        touch /var/log/backup.log
+        crond -f -l 2
+
 volumes:
   gitea_data: