feat: Docker support for L2 server

- Dockerfile for L2 ActivityPub server
- docker-compose.yml for standalone L2
- docker-stack.yml for full swarm deployment (L1+L2+Redis)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,28 @@
+version: "3.8"
+
+services:
+  l2-server:
+    build: .
+    image: git.rose-ash.com/art-dag/l2-server:latest
+    ports:
+      - "8200:8200"
+    environment:
+      - ARTDAG_DOMAIN=artdag.rose-ash.com
+      - ARTDAG_USER=giles
+      - ARTDAG_DATA=/data/l2
+      - ARTDAG_L1=http://l1-server:8100
+    volumes:
+      - l2_data:/data/l2
+    networks:
+      - artdag
+    deploy:
+      replicas: 1
+      restart_policy:
+        condition: on-failure
+
+volumes:
+  l2_data:
+
+networks:
+  artdag:
+    external: true