How to check if Collabora/CODE is running in docker image

I am trying to set up Collabora/CODE with Nextcloud. I have followed this guide but cannot succeed in tying my Apache subdomain to the Docker image.

My question is, how can I check if Collabora is even running correctly inside the Docker image? I am expecting to see some Collabora message of some kind when I browse to the subdomain ( However, when I do that, nothing shows. I would like to know this in order to help me to debug this issue further.