I know exactly what he is talking about. When you run Calibre using the docker image
https://hub.docker.com/r/linuxserver/calibre you are presented with a web interface which is identical to the regular Calibre GUI. I'm not exactly sure how they are doing this, but I believe it runs an html server called Guacamole.
Using the regular Calibre interface, you can drag and drop books into the library. However, when using the docker version, even though the interface looks identical, the web version of the interface does not let you drag and drop books.
I have resorted to using the content service (also part of the docker image) to get my books into the library. It's clunky, but it works.
Pictures:
https://imgur.com/a/UreZBAR