Start your nitrox docker container once to create the necessary folders.
Go to the folder where your config is mounted and open the "saves" folder. You should see a folder with My World.
Nitrox server saves on Windows are usually in: C:\Users\<YourUser>\AppData\Roaming\Nitrox\saves\<YourServerName>
If you don't know how to get to the Nitrox folder, you can open the nitrox app go to your server settings and click on "Open world Folder".
in the folder you will find multiple files and folders, Those are your save files.
Copy all files and folders from the existing save folder to the "My World" folder in your mounted docker volume. Start the nitrox container again, and your server should now load the copied save files.