How to confirm Collabora server is running (outside Nextcloud)? and URL is set in Nextcloud?

I’m trying to debug whether my Collabora server is running as it should. The Docker container seems OK(?). Is there a way from the command line, even a curl request, to confirm that the server is indeed running properly?

Also, where does Nextcloud store the Collabora URL? In the database? Or in a config file? I want to confirm that this is indeed being written. Even though I’m seeing it in the input field, I wanna be sure because I’m seeing errors again (PHP Fatal error: Cannot declare class GuzzleHttp) same as when it was empty.

You can check this with

… if you see a XML list (wopi-discovery) - then your Collabora server works

Thanks. I’m seeing:

# Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request  GET /hosting/discovery.

Reason:  **Error reading from remote server**

Apache/2.4.18 (Ubuntu) Server at Port 443

So I guess I need to debug the apache virtual host config first.