Error response from daemon: driver failed programming external connectivity on endpoint nifty_knuth

$ docker run -p 9999:873 -v /var/tmp/jy003s66t:/data -e USERNAME=admin -e PASSWORD=mysecret axiom/rsync-server
docker: Error response from daemon: driver failed programming external connectivity on endpoint nifty_knuth (9968cc89e7dc503637acf6c7e5a80311ec29268ec0664890e00f0a94a828e20d): Bind for 0.0.0.0:9999 failed: port is already allocated.

Why?

And anyway, more generally, how do you virtualize an rsync server?
Possibly with access via SSH key (without password).

I practically would like to use rsync (client) in this way

$ rsync -ai src localhost:/dest