安装docker-ce，执行https://mirror.tuna.tsinghua.edu.cn/help/docker-ce/

添加docker-ce库以后，执行sudo apt update,报如下错误：

W: GPG error: https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8

运行如下命令，添加公钥解决：

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

命令执行结果：

Executing: /tmp/apt-key-gpghome.WdAbHsAr79/gpg.1.sh --recv-keys --keyserver keyserver.ubuntu.com 7EA0A9C3F273FCD8
gpg: key 8D81803C0EBFCD88: 5 signatures not checked due to missing keys
gpg: key 8D81803C0EBFCD88: public key "Docker Release (CE deb) <docker@docker.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1

重新执行 sudo apt update，可以正常更新