I can confirm that deleting GUAC_USER and GUAC_PASS variables (aka user name, password) and adding the variable password ( set to abc) does work. Normal Calibre operation has resumed.
when you setup Calibre on docker did you map a path to the directory you keep your books in. Calibre has a /books path that you change at inception. Point /books to /mint/user/media, then run the docker.
once started change the library location to /books and you should see your files.
hope this helps