This is a docker image based on zabbix-server-mysql (ubuntu) with a signal-cli preinstalled.
You start this container just like https://hub.docker.com/r/zabbix/zabbix-server-mysql, but you should mount /var/lib/zabbix/.local/share/signal-cli to the host, because it contains all data of the attached account.
You'll still need to configure signal-cli according to https://github.com/AsamK/signal-cli/wiki/Quickstart#set-up-an-account or link an existing account as described in https://github.com/AsamK/signal-cli/wiki/Linking-other-devices-(Provisioning).
To execute these commands you can eighter attach to the container with docker attach <container> or execute commands like docker exec <container> <command>.
There is a good tutorial on how to create the alert script here https://github.com/GKaveH/Zabbix/wiki/Zabbix-alarm-on-Signal#step-6--create-signal-script-for-sending-message-automatically