feat: Docker 패키징 (로컬 테스트 + 홈서버 배포)

- Dockerfile: python:3.12-slim 베이스, headless streamlit
- docker-compose.yml: ADMIN_TOKEN 환경변수, votes.db 영속 볼륨, participants.json read-only mount
- .dockerignore: 빌드 컨텍스트 최소화

테스트:
- docker compose build OK
- 컨테이너 실행 후 http://localhost:8501 200 OK 확인

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,20 @@
+services:
+  vote:
+    build: .
+    image: hackathon-vote:latest
+    container_name: hackathon-vote
+    ports:
+      - "${PORT:-8501}:8501"
+    environment:
+      ADMIN_TOKEN: ${ADMIN_TOKEN:-change-me}
+      VOTE_DB: /data/votes.db
+      PARTICIPANTS: /app/participants.json
+    volumes:
+      # 호스트의 participants.json 변경 즉시 반영 (재배정 시)
+      - ./participants.json:/app/participants.json:ro
+      # 투표 DB는 호스트에 영속 (컨테이너 재시작해도 유지)
+      - vote-data:/data
+    restart: unless-stopped
+
+volumes:
+  vote-data: