修复The following signatures couldn't be verified because the public key is not available

apt-get update后，可能会出现以下问题：

Get:5 http://xxxxxx

Err:5 http://xxxxxx

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY xxxxxxxx（'公钥编码'）

并且伴随出现
E: The repository 'http://xxxxxx‘ 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.

解决办法：
将公钥添加至服务器，即终端中输入

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys '公钥编码'

添加成功后终端输出为
gpg: key '公钥编码': public key "xxxxxx" imported
gpg: Total number processed: 1
gpg: imported: 1