This will build a docker image that runs BitTorrent Sync.
docker build -t btsync .
docker run -d -p 8888:8888 -p 55555:55555 -v /srv/btsync/:/btsync/ btsync
-d run in detached mode
-p expose container port [public-port]:[container-port]
btsync.conf sets the container ports 8888 as the web ui and 55555 as the listening port
If you do not explicitly set a public port, a random open port will be used because the ports are exposed in the Dockerfile
-v mount a local directory in the container [host-dir]:[container-dir]
btsync.conf should be located in a directory mounted to the container directory
/btsync/
More details are available in this tutorial.