6 questions are enough
In one question you exclude the joker. If the people are A, B, C, ask to A:
"Is B a knave or C a knight".
If A says yes, continue with B. If A says no, continue with C.
With the selected person you can find the correct door with a binary search by asking:
"If I'd ask you whether the door to parasise is among 1-15, would you say yes?"
and halving the range appropriately with each question. In total, you need 1 question to identify a non-joker and 5 to identify the door.
6 questions are necessary
You need to identify 1 possibility among 29.
You could in principle halve the number of possibilities of the worst case with each questions and identify the door in 5 questions.
But if the first question goes to the joker, you won't learn anything. The joker can reply yes or no to any question regardless of where the door is. If you think of it, he could reply randomly without even knowing which one is the right door. Therefore you cannot learn anything about the door from the joker.
So, in the worst case, your first question goes to the joker and you still have 29 doors to discriminate. And for this you need 5 more questions. The wost case requires 6 questions.