It's an attack bonus to any ships built there.
Yes, if you build/buy an empty hull/unarmed ship and later upgrade it, it will retain the attack bonus.
(Note also that there is a 25% bonus to any attack to any ships while they are in the orbit of a planet, and that there is a hidden 1% bonus to both attack and defense that does not show up in your racial abilities to all ships, regardless of...anything.)
