ubuntu添加源报错The following signatures couldn't be verified because the public key is not available

本身是一个非常简单的问题，这里简单做个记录，容器里面一般会有安装一些软件调试的需求，换个源之后会出现导入gpg的错误。
这里从改源开始

root@machine-learn:/etc/apt# echo "deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse" > sources.list
root@machine-learn:/etc/apt#  apt update
Get:1 http://mirrors.aliyun.com bionic InRelease [242 kB]
Ign http://mirrors.aliyun.com bionic InRelease
Get:2 http://mirrors.aliyun.com bionic/main amd64 Packages [1344 kB]
Get:3 http://mirrors.aliyun.com bionic/restricted amd64 Packages [13.5 kB]
Get:4 http://mirrors.aliyun.com bionic/universe amd64 Packages [11.3 MB]
Get:5 http://mirrors.aliyun.com bionic/multiverse amd64 Packages [186 kB]                                                                                                                                                                  
Fetched 13.1 MB in 9s (1446 kB/s)                                                                                                                                                                                                          
Reading package lists... Done
Building dependency tree       
Reading state information... Done
118 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: GPG error: http://mirrors.aliyun.com bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32

# 报了个错误。开始导入key，key的这个值取决于上面的提示
root@machine-learn:/etc/apt# gpg --keyserver keyserver.ubuntu.com --recv 3B4FE6ACC0B21F32 
gpg: directory `/root/.gnupg' created
gpg: new configuration file `/root/.gnupg/gpg.conf' created
gpg: WARNING: options in `/root/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/root/.gnupg/secring.gpg' created
gpg: keyring `/root/.gnupg/pubring.gpg' created
gpg: requesting key C0B21F32 from hkp server keyserver.ubuntu.com
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key C0B21F32: public key "Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
root@machine-learn: /etc/apt# gpg --export --armor 3B4FE6ACC0B21F32 |apt-key add -
OK


# 接下来继续执行apt update，随后就可以下载安装工具了
root@machine-learn:/etc/apt# apt update