docker: hourly prune of objects older than 7 days, root mail only when the prune fails

docker/files/docker-prune

@@ -0,0 +1,12 @@
+#!/bin/bash
+# Silent on success: cron mails all output to root, so mail arrives only when the prune fails.
+set -u
+
+output=$(docker system prune -af --filter "until=168h" 2>&1)
+rc=$?
+
+if [ "$rc" -ne 0 ]; then
+    printf '%s\n' "$output"
+    echo "docker system prune exited with rc=$rc"
+    exit "$rc"
+fi