GPG error: https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.4 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 656408E390CFB1F5

最近部署mongodb的错误，解决方法：

apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 656408E390CFB1F5

类似的错误，也可以使用相同方法解决；