I think the leaves would add lovely stuff to compost as the roots are very deep and draw nutrients up from the depths. However, you would want to be very careful not to include any root or seeds - they add bulk to the bonfire

. The root can be used as a dyestuff, but I think it only gives the ubiquitous yellow
I have a little book which gives 101 uses for nettles, but I don't think there's a similar one for docken.