The repository 'http://cdn.debian.net/debian stretch Release' is not signed.

/*********************************************************************************
 *   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