by default the service shares everything under /shared
For quick and easy sharing, mount your desired folder into /shared in the container like so:
docker run -t -v /whatever/you/want:/shared docker-onionshare
the default behavior is to connect to TOR using the bundled connector and share everything in the /shared folder and stop the service after one download.
Credit goes to micahflee for the excellent onionshare
https://github.com/micahflee/onionshare