infra: add Docker setup for portable deployment

Dockerfile (Python 3.12 slim) + docker-compose (backend + nginx).
Backend on port 8000 inside container, nginx proxies API and serves
frontend static. SQLite persisted in named volume. Nginx listens on
127.0.0.1:8080 — external SSL handled by host reverse proxy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,23 @@
+services:
+  backend:
+    build: .
+    restart: unless-stopped
+    env_file: .env
+    environment:
+      DB_PATH: /data/baton.db
+    volumes:
+      - db_data:/data
+
+  nginx:
+    image: nginx:alpine
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:8080:80"
+    volumes:
+      - ./frontend:/usr/share/nginx/html:ro
+      - ./nginx/docker.conf:/etc/nginx/conf.d/default.conf:ro
+    depends_on:
+      - backend
+
+volumes:
+  db_data: