问题： 

sudo apt update

报错 ：

GPG error: https://download.docker.com/linux/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8

一般遇到NO_PUBKEY只需要添加这个key就行：

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7EA0A9C3F273FCD8

但是对于这个docker的key却不行。

解决办法

First, add key to /usr/share/keyrings/docker-archive-keyring.gpg by command:

sudo curl -fsSL "https://download.docker.com/linux/ubuntu/gpg" | sudo gpg --dearmor --yes -o /usr/share/keyrings/docker-archive-keyring.gpg

PS For some the part gpgp --dearmor was useful.

Second, update permissions for it:

sudo chmod a+r /usr/share/keyrings/docker-archive-keyring.gpg

Then，

sudo apt update

参考：

Docker: How to solve the public key error in ubuntu while installing docker - Stack Overflow

解决GPG error: The following signatures couldn't be verified because the public key is not available_追求卓越583的博客-CSDN博客