Three volumes are configured for this repo:

/volumes/config: where NZBGet will keep it's config files.
/volumes/media: where your media is located.
/volumes/downloads: where your NZBGet will put downloaded files.

Easiest way to run:

sudo docker run --restart always -d --name="transmission" -h transmission -v /etc/localtime:/etc/localtime:ro -v <path_to_your_config_on_host>:/volumes/config -v <path_to_your_downloads_on_host>:/volumes/downloads -v <path_to_your_media_on_host>:/volumes/media -p 12345:12345 -p 12345:12345/udp -p 9091:9091 jrarseneau/transmission

Default username is transmission and password is transmission. In order to change, edit the appropriate password entry in settings.json which will be copied to your local config directory.