我创建了一个 Ubuntu 18.04 VM，我想在上面安装 docker。我按照以下说明操作：https://docs.docker.com/engine/install/ubuntu/。
我在这一步遇到了麻烦：
# sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
Hit:1 http://ubuntumirror.it.att.com/ubuntu bionic InRelease
Hit:2 http://ubuntumirror.it.att.com/ubuntu bionic-updates InRelease
Hit:3 http://ubuntumirror.it.att.com/ubuntu bionic-security InRelease
Ign:4 https://download.docker.com/linux/ubuntu bionic InRelease
Err:5 https://download.docker.com/linux/ubuntu bionic Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 135.28.13.29 8080]
Reading package lists... Done
E: The repository 'https://download.docker.com/linux/ubuntu bionic Release' does not have a Release file.
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.
我现在该怎么办？