1. Upload your map to Dropbox or another online storage solution that gives you a link.
2. Open the File Manager
3. Navigate to /server/docker/cfg/server.cfg
(if the file doesn't exist, right click and create the file).
4. Right click server.cfg
and click Edit
5. Add this line with a link to your map. *Note: For dropbox links make sure it ends with ?dl=1.
levelurl "https://link-to-your-map/...."
6. Restart your server and it will download and load the custom map