jvb: adds autoscaler sidecar hook to signal final shutdown (#1809)

* jvb: adds autoscaler sidecar hook to signal final shutdown

* cleaner ports

jvb/rootfs/opt/jitsi/shutdown.sh

@@ -1,4 +1,8 @@
 #!/usr/bin/with-contenv bash
+# notify the sidecar of imminent shutdown
+PORT=${AUTOSCALER_SIDECAR_PORT:-6000}
+curl -d '{}' -v 0:$PORT/hook/v1/shutdown
+sleep 10
 
 # shutdown everything
 s6-svscanctl -t /var/run/s6/services