I know I'm late to the game but.... I LOVE IT
Last year I'd bought a QNAP NAS for backups etc at home, installed Ubooquity and had it serving up from a replica of my Calibre library from my PC (with a file sync program on the PC ensuring the replica was current).
Last week I tried installing a QPKG version of COPS, and while it basically worked, things like downloading an ePub to Moon+ Reader failed; some minor corruption seemed to happen.
Yesterday I thought maybe I should try again with a Docker version, so added the linuxserver/cops package. Install went great but it kept telling me unable to read database, even though it was in the correct location
Today experimented with having the container run as a different userid (the one who owns all the files in the library). WOOT.. It works GREAT!