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>

Dockerfile

@@ -0,0 +1,20 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY app.py assign_teams.py ./
+COPY participants.json ./
+
+EXPOSE 8501
+
+ENV VOTE_DB=/data/votes.db \
+    PARTICIPANTS=/app/participants.json
+
+CMD ["streamlit", "run", "app.py", \
+     "--server.address=0.0.0.0", \
+     "--server.port=8501", \
+     "--server.headless=true", \
+     "--browser.gatherUsageStats=false"]