The dashboard now requires authentication to view, see announcement from a couple of months back.
Is there any way you can monitor the services on the Pi-hole directly. eg check for the presence of pihole-FTL running (eg via SSH using a key)? If not then you could use the chronometer web page approach I posted and simplify it even more (see below). I don't know Docker well so adjust if needed.
Simplified
On the Pi-hole create a symbolic link
sudo ln -s /home/pi/status.txt /var/www/html/pihole/status.txt
Open pi's crontab
crontab -e
Add in the entry below to update the status every 1 minute, and save.
* * * * * bash -l pihole -c -e | awk '/Pi-hole/{print $1,$2}' > /home/pi/status.txt
You can now monitor the text at
http://<your pihole ip>/pihole/status.txt
It will be updated every minute via cron.
Security note
Since you're linking to a file outside the webroot, please do make sure that the Pi-hole is secure and up to date and accessible to only those who should have access. You'll need to make your own security risk assessment of using this approach. In terms of who can access the info without authentication it will be no different to what you had before.
Monitoring for the word "Active" seems a bit risky, in that if the Pi-hole fails then you won't be able to reach a page to monitor. Your monitoring logic would have to make sure it treats that as the service being down. But then if just the web component fails it would flag Pi-hole as down even if DNS was up. Worth going through some example scenarios and assessing the possible outcomes of this approach.
It's possible that you'll read the file exactly as it's being updated and get a blank result, potentially causing a false alarm. Add a few seconds delay to your checking so it's not exactly on the minute so writing and checking don't coincide.
Not sure if the link in www will survive an update; it may be overwritten. Check on next available update.
(Edited to include exact crontab entry needed to work properly and add extra note)