docker给apt-get换国内源

docker在使用apt-get的时候非常缓慢，尝试网上很多方法换源都导致失败。
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最后尝试成功：

cp /etc/apt/sources.list /etc/apt/sources.list.bak #备份原来的文件
echo "" > /etc/apt/sources.list #更换163源
echo "deb http://mirrors.163.com/debian buster main" >> /etc/apt/sources.list
echo "deb http://mirrors.163.com/debian-security buster/updates main" >> /etc/apt/sources.list
echo "deb http://mirrors.163.com/debian buster-updates main" >> /etc/apt/sources.list

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