/var/lib/apt/lists/* 这个目录下的都是些什么文件，为什么每次新建Dockerfile都要删除呢？
主要是减少最终镜像的大小。通常是可以减少21M以上。
官方文档对此有描述：
In addition, when you clean up the apt cache by removing /var/lib/apt/lists it reduces the image size, since the apt cache is not stored in a layer. Since the RUN statement starts with apt-get update, the package cache is always refreshed prior to apt-get install.