Interesting question. I think it depends whether the ball is considered in the bunker. Per the definition below, I think stairs leading into a bunker are not considered part of a bunker and therefore you can get free relief and drop outside the bunker without penalty.
If the ball is considered in the hazard and taking a drop in the bunker would put you closer to the hole then you'd have to drop outside the bunker with a one shot penalty.
USGA definition:
A "bunker" is a hazard consisting of a prepared area of ground, often a hollow, from which turf or soil has been removed and replaced with sand or the like.
Grass-covered ground bordering or within a bunker, including a stacked turf face (whether grass-covered or earthen), is not part of the bunker. A wall or lip of the bunker not covered with grass is part of the bunker. The margin of a bunker extends vertically downwards, but not upwards.
A ball is in a bunker when it lies in or any part of it touches the bunker.