/*********************************************************************************
* The repository 'http://cdn.debian.net/debian stretch Release' is not signed.
* 说明：
* debootstrap获取Debian文件系统的时候报错。
*
* 2018-3-2 深圳 宝安西乡 曾剑锋
********************************************************************************/
一、参考文档：
1. Debian 9 (stretch) - why 'apt-get update' cannot write tmp files?
https://www.linuxquestions.org/questions/linux-newbie-8/debian-9-stretch-why-%27apt-get-update%27-cannot-write-tmp-files-4175619721/
二、错误现象：
W: No sandbox user '_apt' on the system, can not drop privileges
W: GPG error: http://cdn-fastly.deb.debian.org/debian stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010 NO_PUBKEY EF0F382A1A7B6500
W: The repository 'http://cdn.debian.net/debian stretch Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
三、解决办法：
deb http://httpredir.debian.org/debian stretch main contrib non-free
