The following signatures couldn‘t be verified because the public key is not available:

Ubuntu改镜像源，比如我要安装K8s的时候，执行命令：

sudo apt-get update

提示了我这个：

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEXXXXXXXX NO_PUBKEY 8BXXXXXXXX
GPG error: https://mirrors.aliyun.com/kubernetes/apt kubernetes-xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEXXXXXXXX NO_PUBKEY 8BXXXXXXXX

解决方法：
执行命令：

	sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com XXXX