chore(docker): env2 runs its own image tag, leader-auto-down overlay, drill targets env2 (#116)

- docker/build.sh: SCADABRIDGE_IMAGE picks the tag (default scadabridge:latest).
- docker-env2/docker-compose.yml: node image is ${SCADABRIDGE_IMAGE:-scadabridge:latest},
  so env2 can run its own build without retagging the primary rig's image.
- docker-env2/docker-compose.leader-auto-down.yml: opt-in overlay for both env2 pairs,
  mirroring docker/docker-compose.leader-auto-down.yml. Shipped default stays auto-down.
- docker/failover-drill.sh: DRILL_ENV=primary|env2 (container prefix, Traefik :9100,
  pair site-x).

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

docker-env2/docker-compose.leader-auto-down.yml

@@ -0,0 +1,28 @@
+# Opt-in overlay: run env2's central pair and its site-x pair on the in-repo
+# `leader-auto-down` downing provider (LeaderAutoDowningProvider, ScadaBridge#106)
+# instead of the shipped default `auto-down`. env2 is the soak environment for the
+# #116 rollout; mirrors docker/docker-compose.leader-auto-down.yml.
+#
+# The shipped default is NOT changed by this file: the mounted appsettings still say
+# `auto-down`; this overlay layers an environment override on top (env wins over JSON).
+#
+# A downing-provider change must reach both nodes of a pair at the same time - never
+# roll it one node at a time. Apply (from the main checkout, both pairs together):
+#
+#   docker compose -f docker-env2/docker-compose.yml -f docker-env2/docker-compose.leader-auto-down.yml \
+#     up -d --force-recreate --no-deps central-a central-b site-x-a site-x-b
+#
+# Revert to the default by re-running the same command without the overlay file.
+services:
+  central-a:
+    environment:
+      ScadaBridge__Cluster__SplitBrainResolverStrategy: "leader-auto-down"
+  central-b:
+    environment:
+      ScadaBridge__Cluster__SplitBrainResolverStrategy: "leader-auto-down"
+  site-x-a:
+    environment:
+      ScadaBridge__Cluster__SplitBrainResolverStrategy: "leader-auto-down"
+  site-x-b:
+    environment:
+      ScadaBridge__Cluster__SplitBrainResolverStrategy: "leader-auto-down"