Calibre-web is a web app providing a clean interface for browsing, reading and downloading eBooks using an existing Calibre database. It is also possible to integrate google drive and edit metadata and your calibre library through the app itself.
The Calibre-Web docker is only a web front end to the actual Calibre application/database itself. You still need a Calibre metadata.db file for Calibre Web to function. To get this, you have to install Calibre somewhere and you can move the metadata.db file into your /storage folder.