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.

.dockerignore

@@ -0,0 +1,19 @@
+**/bin/
+**/obj/
+**/logs/
+**/data/
+**/.DS_Store
+.git/
+.vs/
+.idea/
+docker/
+infra/
+tests/
+docs/
+AkkaDotNet/
+*.md
+site_events.db
+TestResults/
+*.trx
+__pycache__/
+*.pyc