adding cronjob file

files/docker-system-prune-all.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+LOG_FILE="/var/log/docker-system-prune.log"
+echo "===== $(date '+%Y-%m-%d %H:%M:%S') =====" >> "$LOG_FILE"
+# "system" prune, because pruning images only leaves a lot of garbage around
+docker system prune -a --force >> "$LOG_FILE" 2>&1
+
+echo "" >> "$LOG_FILE"
+exit 0