You can think of it as a shortening of this phrase: Přeji ti dobrou noc. (I wish you a good night.) The verb needs to be followed by accusative case as was correctly mentioned by BoneheadBass. Dobrou is the accusative form of dobrá.

"Noc" is feminine and in this sentence it's in the accusative case. So the adjective "dobrá" becomes "dobrou." Hope this helps!

Think of the accusative case as the "direct object" case.