The requirement is that

• The door should be lockable and unlockable from both the inside and the outside, and
• If the door is locked from the inside, it should not be unlockable from outside, right?

So you can have a door with a lock on the outside and a separate bolt / latch (or two) on the inside. There is a risk that someone with a duplicate key can lock you in from outside (a common prank) so some versions have a lever on the inside that can unlock the door even if it is locked from outside.