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>

Dockerfile

@@ -0,0 +1,19 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+# Install dependencies
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application
+COPY . .
+
+# Create data directory
+RUN mkdir -p /data/l2
+
+ENV PYTHONUNBUFFERED=1
+ENV ARTDAG_DATA=/data/l2
+
+# Default command runs the server
+CMD ["python", "server.py"]