[HOW TO] The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

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sky@zabbix4-srv-01:/opt/packages$ sudo apt-get updateGet:1 http://repo.zabbix.com/zabbix/4.0/ubuntu xenial InRelease [7096 B]Hit:2 http://security.ubuntu.com/ubuntu xenial-security InReleaseGet:3 http://ppa.launchpad.net/ondrej/php/ubuntu xenial InRelease [23.9 kB]Hit:4 http://us.archive.ubuntu.com/ubuntu xenial InReleaseGet:5 http://repo.zabbix.com/zabbix/4.0/ubuntu xenial/main Sources [1196 B]Ign:3...