This is a common problem for all volkswagens.  It has to do with the
regulator on the alternator not being able to pump out over 12 volts until
the voltage spike tickles it.  very technical, I know.  Don't worry about the
light.  Rev the engine until the light goes out then check the voltage at the
battery, with no accessories on it should be 13-14.5 volts.  Too low and it
wont charge your battery, too high and it will boil your battery.