Icecast itself is completely free, open-source software — the Icecast software itself is completely free and open source; what you pay for with hosting is the server, bandwidth, and management. Since you already have the Ubuntu Docker host running at 172.16.12.243, you have that infrastructure covered — this costs you nothing beyond what you're already paying, no listener caps, no bitrate limits, no third party holding your stream. This is genuinely the same shape of project as your Zabbix stack: a docker-compose file, a couple of ports opened, done in under an hour