This container runs a headless Spotify player that can be remote-controlled by any Spotify app. Audio is output to a pipe, which can be consumed in another container or the host system by alsa, pulseaudio, forked-daapd (to Airplay), snapserver, etc.
This requires a Spotify premium account, but does not require a Spotify developer key or libspotify binary.
The process run is librespot, an open source client library for Spotify.
Play audio to /tmp/spotify-pipe:
docker run -d \
-v /tmp/spotify-pipe:/data/fifo
-e SPOTIFY_NAME=Docker \
-e SPOTIFY_USER=... \
-e SPOTIFY_PASSWORD=... \
kevineye/librespot