If you add new volume mappings to the docker container for /Completed and /Incomplete, then you can point them, individually, to anywhere accessible from your server, giving you total flexibility.
My volume mappings:
'LeechTorrents' are those currently downloading. On completion, deluge automatically moves them to 'Torrents'. I then do some manual processing to extract the files I need after which I manually move the torrents to 'ReadyTorrents' from within deluge.