On the Spotify help page it says,
" Spotify has done its best to make sure Spotify Mobile works according to
the standards that are available on the market. However, some headsets,
docking stations and other accessories might not work correctly with
Spotify. Some of these are designed to only work with the device's built
in player. For some docking stations updating the firmware can help
when playing music from Spotify while docked."
So sounds like its pot luck whether it works with a dock or not!