Even though the suggestion is a bit older and hasn't found any supporters yet, Cloudron still lacks server monitoring software!
My primary concern would be to monitor other servers such as web or database servers. And in more depth than just uptime. As a nice side effect, the Cloudron server could perhaps also be monitored.
A good list of features can be found at https://checkmk.com/product/server-monitoring-software:
Scalable server monitoring software for Sysadmins and DevOps
Ensure peak server performance for your infrastructure with the best server monitoring. Say goodbye to preventable outages, late nights, or weekend emergencies.
Monitor any type of server (web, mail, database servers etc.)
Flexible server monitoring through the use of more than 2,000 smart checks
Easy-to-use configuration, auto-discovery and network mapping
Native agents for Windows, Linux and many more operating systems
Agentless monitoring with SNMP and TCP/UDP (FTP, LDAP, IMAP etc.)
API-based checks based on HTTP/XML, SSH or TELNET
Measurement intervals as short as <1 minute
There is a Docker installation: https://docs.checkmk.com/latest/en/introduction_docker.html
In addition is the Checkmk Raw Edition free and 100% open-source.
See also https://github.com/Checkmk/checkmk