fix(dns): retry configurable resolvers

Treat resolver transport failures as inconclusive checks so they do not open outage alerts.

packaging/systemd/worker.env.example

@@ -14,3 +14,6 @@ WORKER_CLUSTER_PEERS=
 WORKER_CLUSTER_DATA_DIR=/var/lib/rsmon-worker/cluster
 WORKER_CLUSTER_BOOTSTRAP=false
 WORKER_RELEASE_URL=
+# Comma-separated IPv4/IPv6 recursive resolvers used by DNS checks. Empty
+# uses 188.93.17.19, 188.93.16.19, and our IPv4-only resolver 82.117.245.122.
+RSMON_DNS_RESOLVERS=188.93.17.19,188.93.16.19,82.117.245.122