I've been using uptime Kuma recently and it's great but works better outside of docker.
Inside docker I'd get a lot of false down positives from I assume docker throttling the checks.
Plus it works with email, telegram, and matrix chat alerts. I monitor all my clients sites with it, and it's bullet proof behind caddy.
I just do web hosting for clients sites and use Kuma to monitor uptime and SSL certificates.
Ive got multiple Kuma's running as well.