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
4. Right click to edit server.cfg
5. Add this line with a link to your map. *For dropbox links make sure it ends with ?dl=1.
levelurl "https://link-to-your-map/...."
6. Restart your server and it should download and load the custom map.