Ok, I got it working. You need to:
1. change the "-v /storage/.config/dockers/transmission/downloads" line to your external path
(for example: -v /media/MYDRIVE/MyDownloads:/downloads \)
*and*
2. in the web UI, set the "Download to:" field to /downloads
You are my HERO!
The only thing I had to do was to change the "download to:"