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/build.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+
+echo "=== ScadaLink Docker Build ==="
+
+# Ensure the shared network exists
+if ! docker network inspect scadalink-net >/dev/null 2>&1; then
+    echo "Creating scadalink-net network..."
+    docker network create scadalink-net
+fi
+
+# Build from repo root (so COPY paths in Dockerfile resolve correctly)
+echo "Building scadalink:latest image..."
+docker build \
+    -t scadalink:latest \
+    -f "$SCRIPT_DIR/Dockerfile" \
+    "$REPO_ROOT"
+
+echo "Build complete: scadalink:latest"