Lets Encrypt plugin - What web root?

I have OMV 4 running with a ton of services running in Docker... And I don't really know what to put in the plugin for the webroot. I tried "/var/www/openmediavault" but that failed the challenges. I'm running Plex, HASSIO, qBittorrent, Radarr and several others. Any help would be appreciated.

Also, as a suggestion, maybe a future version of the plugin could run it's own challenge without needing to ask for a web root and then as an option a user can specify the webroot if they are trying to use it for something specific.

Update - also I should mention that I have nothing running on port 80 just for the Let's Encrypt plugin to run.

Yes the port is forwarded. Let's Encrypt also works great in couple of addons in HASSIO, so I know that my port forwarding works. Nothing on that side of things has changed.

I'm trying to get the plugin working so that I can easily use the same cert over the entire system easily. I can declare it as a volume in other Docker images. I'm sure that I could do that with a Docker image too, but this is how I'm experimenting with things at the moment. I'm trying to solve the broken cert warning in the browser for all my different services and I'd like for it to just work. Instead I'm seeing an old cert in some windows and applying on cert across the whole system, starting at the root with OMV seems like a reasonable idea.

Yes I run HASSIO which is installed via the curl command you referenced.