feat: add Docker deployment configuration

添加 Docker 部署支持：
- Dockerfile: 多阶段构建，包含 ffmpeg 依赖
- docker-compose.yml: 配置端口映射和持久化目录
- .dockerignore: 优化构建上下文

Co-Authored-By: AI

docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  bilidown:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    image: bilidown:latest
+    container_name: bilidown
+    restart: unless-stopped
+    ports:
+      - "8098:8098"
+    volumes:
+      # 数据库持久化
+      - ./data:/app/data
+      # 下载目录
+      - ./downloads:/app/downloads
+    environment:
+      - TZ=Asia/Shanghai
+      - GIN_MODE=release
+    healthcheck:
+      test: ["CMD", "wget", "-q", "--spider", "http://127.0.0.1:8098/"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 10s