Use Umbrel DNS for Datum polling

.env.example

@@ -13,11 +13,9 @@ LOG_LEVEL=info
 # STATIC_DIR=
 
 # ---- Datum gateway (the Umbrel app we're polling) ----
-# Fallback URL. In Docker deployment, DATUM_DOCKER_* below lets the API discover
-# Datum's current container IP before polling.
-DATUM_URL=http://datum_datum_1:21000
-DATUM_DOCKER_CONTAINER=datum_datum_1
-DATUM_DOCKER_NETWORK=umbrel_main_network
+# Datum's service alias on Umbrel's shared app network. Override if your Umbrel
+# install exposes a different DNS name.
+DATUM_URL=http://datum:21000
 DATUM_ADMIN_USER=admin
 DATUM_ADMIN_PASSWORD=
 # How often to scrape /clients (ms). Datum updates per-worker hashrate every