refactor: 优化缓存目录配置和Dockerfile结构

修改缓存目录配置以支持环境变量覆盖，简化Dockerfile并优化镜像构建步骤
移除冗余权限设置，使用更精简的基础镜像

Dockerfile

@@ -1,34 +1,20 @@
-FROM mcr.microsoft.com/devcontainers/javascript-node:22
+FROM node:22-slim
 
-# Install ffmpeg for video thumbnail support
 RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
 ENV NODE_ENV=production
+ENV CACHE_DIR=/app/.cache
 WORKDIR /app
 
-# Use Corepack to activate pnpm from package.json's packageManager field
 COPY package.json ./
 RUN corepack enable && pnpm --version && pnpm install --prod
 
-# Copy application sources
+# 只复制必要源代码，避免把缓存等无关内容塞进镜像
 COPY . .
 
-# Run as non-root for security
-RUN chown -R node:node /app
-USER node
+RUN useradd -m appuser && mkdir -p /app/.cache && chown -R appuser:appuser /app
+USER appuser
 
-# 定义.cache目录. 支持用户自定义
-ENV CACHE_DIR=/app/.cache
-
-# 确保.cache目录存在
-RUN mkdir -p $CACHE_DIR && chown -R node:node $CACHE_DIR
-
-# 确保.cache目录可写
-RUN chmod -R 777 $CACHE_DIR
-
-# index listens on 9520
 EXPOSE 9520
-
-# Default command
-CMD ["node", "index.js"]
+CMD ["node", "index.js"]