The container runs mopidy, which pipes it's audio output to a pulseaudio sink, from where it is sent via an RTP Broadcast on 22.214.171.124:45678.
Plugins for Spotify and a Webinterface are already included.
While this can run on a Raspberry Pi 1, a Raspberry Pi 2 is recommended.
docker run --net=host -e USER='YOUR SPOTIFY USERNAME' -e PASS='YOUR SPOTIFY PASSWORD' malted/mopidy-pulseaudio-server
You can listen to the stream for example with
Navigate your Browser to YOUR_IP:6680 to control mopidy via its webinterface