无法在我的 ubuntu 上安装 docker，因为 apt-get update 中出现“NO_PUBKEY 7EA0A9C3F273FCD8”错误

我正在尝试在 Ubuntu 20.04 LTS 上安装 Docker。我正在使用官方文档。我以前做过，没有问题。

这是运行后的错误sudo apt-get update：

Hit:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 http://packages.microsoft.com/repos/code stable InRelease                                                       
Get:3 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]                                             
Hit:4 http://ir.archive.ubuntu.com/ubuntu focal InRelease                   
Hit:5 http://ir.archive.ubuntu.com/ubuntu focal-updates InRelease              
Get:6 https://download.docker.com/linux/ubuntu focal InRelease [52.1 kB]       
Hit:7 http://ir.archive.ubuntu.com/ubuntu focal-backports InRelease                        
Err:6 https://download.docker.com/linux/ubuntu focal InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8
Reading package lists... Done      
W: GPG error: https://download.docker.com/linux/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8
E: The repository 'https://download.docker.com/linux/ubuntu focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

我看到了如何修复 GPG 错误“NO_PUBKEY”？但我无法解决问题。