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/site-a-node-b/appsettings.Site.json

@@ -0,0 +1,54 @@
+{
+  "ScadaLink": {
+    "Node": {
+      "Role": "Site",
+      "NodeHostname": "scadalink-site-a-b",
+      "SiteId": "site-a",
+      "RemotingPort": 8082
+    },
+    "Cluster": {
+      "SeedNodes": [
+        "akka.tcp://scadalink@scadalink-site-a-a:8082",
+        "akka.tcp://scadalink@scadalink-site-a-b:8082"
+      ],
+      "SplitBrainResolverStrategy": "keep-oldest",
+      "StableAfter": "00:00:15",
+      "HeartbeatInterval": "00:00:02",
+      "FailureDetectionThreshold": "00:00:10",
+      "MinNrOfMembers": 1
+    },
+    "Database": {
+      "SiteDbPath": "/app/data/scadalink.db"
+    },
+    "DataConnection": {
+      "ReconnectInterval": "00:00:05",
+      "TagResolutionRetryInterval": "00:00:10",
+      "WriteTimeout": "00:00:30"
+    },
+    "StoreAndForward": {
+      "SqliteDbPath": "/app/data/store-and-forward.db",
+      "ReplicationEnabled": true
+    },
+    "Communication": {
+      "CentralActorPath": "akka.tcp://scadalink@scadalink-central-a:8081/user/central-communication",
+      "DeploymentTimeout": "00:02:00",
+      "LifecycleTimeout": "00:00:30",
+      "QueryTimeout": "00:00:30",
+      "TransportHeartbeatInterval": "00:00:05",
+      "TransportFailureThreshold": "00:00:15"
+    },
+    "HealthMonitoring": {
+      "ReportInterval": "00:00:30",
+      "OfflineTimeout": "00:01:00"
+    },
+    "SiteEventLog": {
+      "RetentionDays": 30,
+      "MaxStorageMb": 1024,
+      "PurgeScheduleCron": "0 2 * * *"
+    },
+    "Notification": {},
+    "Logging": {
+      "MinimumLevel": "Information"
+    }
+  }
+}