If an action card turns a planet neutral, are the space docks and PDS units on it destroyed?
The Space Docks and PDS units are destroyed.

From page 3 of the official FAQ version 2.3 


  Q: If a planet contains one Ground Force, one Space Dock, and one PDS,
  and “Local Unrest” is played on that planet, are the Space Dock and
  PDS destroyed? 
  
  A: Yes. The Space Dock and PDS would be destroyed due
  to the planet changing ownership (becoming neutral).


And for good measure, on page 12 of the rulebook:


  If a PDS unit lands alone on a neutral or enemy planet, it is
  immediately destroyed.