Set up with minimal changes not working

[Syndicoon]

I used this container as-is. I changed the hostname per the instructions in the .env file.
I opted not to make changes to the docker-compose.yml file to make this as simple as possible.
My DNS was set to redirect calls to bookmarks.my.hostname to my home server address.
This was done a day before I installed this to make sure the DNS was updated.
The server is Ubuntu, and the UFW firewall has a rule set to allow traffic on port 8080.
I use caddy as a stand-alone server (not a docker container) on my server.
I added the following to the Caddyfile and restarted Caddy:

bookmarks.my.hostname {
reverse_proxy localhost:8080
}
NOTE: This setup works with the following containers: yacht, calibre-web without issues.
I understand that this is a backend API and doesn't have a built-in web GUI. One is supposed to install the xbrowsersync
plugin and then select to point to their home server.
Did all that using https://bookmarks.my.hostname and https://bookmarks.my.hostname:8080 both failed.

I am new to Docker, and I am probably overlooking some simple configuration somewhere.
Or worse, not understanding how to access this backund API correctly.
Any hints, suggestions, or pointers, would be appreciated.