Necro-ing this thread as I have a working answer that may help people (like me) when I was searching for an answer.
I am adding Ubooquity as a subdomain (books) of my domain (website.com). The intended outcome was to go to books.website.com and reach my ubooquity server on a docker container managed by Nginx proxy manager.
Then go to the Custom Location tab. Here we will add the details of how Nginx make the correct connection to Ubooquity.
Finally, on the SSL tab, if you are using SSL, add your server certificate here. I usually check Force SSL and HTTP/2 Support.
There is one little glitch that I can't quite figure out yet is when you browse to the main domain (books.website.com) it forwards like twenty /ubooquity/ levels then the port number /:2202/. Still works and goes away when you navigate into the main library.
Hope this helps!
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