Because zabbix-agent2 in this container is running by the user without permissions to docker socket. There is a possibility to run this, but You must have basic knowledge about linux shell systems. You must check GID of docker group on Host (Unraid Server). Log in to zabbix-agent2 container as root user (default  running "Console" from Unraid GUI made it but as zabbix user) Add group docker (with same gid as on host/unraid server) inside zabbix-agent2 container. Add user zabbix to this