Knock Knock / Who’s there? / Doris!
Oh - it looks like we are back to bad knock-knock jokes. Oh well.
Doris locked that’s why I am knocking!
Doris sounds like Door is, as in "[the] door is locked" The jokester is answering the wrong question - the question was Doris who, but the jokester answered, "why are you knocking on my door?"
I guess that this is supposed to be a pun. It's not very good.