use find instead of mc for pruning local backups

test/compose/docker-compose.yml

@@ -25,6 +25,10 @@ services:
       AWS_S3_BUCKET_NAME: backup
       BACKUP_FILENAME: test.tar.gz
       BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
+      BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-7}
+      BACKUP_FORCE_PRUNE: ${BACKUP_FORCE_PRUNE:-}
+      BACKUP_PRUNING_LEEWAY: 5s
+      BACKUP_PRUNING_PREFIX: test
     volumes:
       - ./local:/archive
       - app_data:/backup/app_data:ro