Are specific rewards locked behind certain doors?
The keys you find throughout the game can be used to unlock doors within your sanctuary. Do the doors already have particular items within them, or is your reward random?
Solution 1:
No, rewards there are random. It's possible to make save just before opening to test this out (and to roll better rewards).
However, their content depends on your game progress. For example, it's possible to get magnum ammo from lockers once you collected the magnum itself.
See this article for list of all possible rewards.