feat: add Docker infrastructure for 8-node cluster topology (2 central + 3 sites)

Multi-stage Dockerfile with NuGet restore layer caching, per-node appsettings
with Docker hostnames, shared bridge network with infra services, and
build/deploy/teardown scripts. Ports use 90xx block to avoid conflicts.

docker/teardown.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+
+echo "=== ScadaLink Docker Teardown ==="
+
+echo "Stopping ScadaLink application containers..."
+docker compose -f "$SCRIPT_DIR/docker-compose.yml" down
+
+echo ""
+echo "Teardown complete."
+echo "Site data (SQLite DBs) and logs are preserved in node directories."
+echo ""
+echo "To also remove persistent data:"
+echo "  rm -rf $SCRIPT_DIR/site-a-node-a/data $SCRIPT_DIR/site-a-node-b/data"
+echo "  rm -rf $SCRIPT_DIR/*/logs"