Use Umbrel Docker DNS for Datum

.env.example

@@ -12,13 +12,13 @@ LOG_LEVEL=info
 # to ../web/dist relative to the running api bundle.
 # STATIC_DIR=
 
+# Host port exposed by the Portainer stack.
+GASHBOARD_PORT=1337
+
 # ---- Datum gateway (the Umbrel app we're polling) ----
-# Portainer on Umbrel runs stacks inside Docker-in-Docker with host networking.
-# Use Datum's real Umbrel Docker IP. Do not use datum/datum_datum_1 DNS and do
-# not use host port 21000, which is Umbrel's auth proxy.
-# Find it with:
-# docker inspect -f '{{.NetworkSettings.Networks.umbrel_main_network.IPAddress}}' datum_datum_1
-DATUM_URL=http://10.21.0.11:21000
+# The stack joins Umbrel's real Docker network, so use Datum's container name.
+# Do not hard-code Datum's container IP; it can change after reboot.
+DATUM_URL=http://datum_datum_1:21000
 DATUM_ADMIN_USER=admin
 DATUM_ADMIN_PASSWORD=
 # How often to scrape /clients (ms). Datum updates per-worker hashrate every