Finding Dory: 26 Easter Eggs & References You Must See
18. Lava
And proving that Pixar don't just "lava" their most recent short, there's also a nod to short film Lava, which played with Inside Out.
In the same scene that features the Piper poster, there's a fridge magnet on a small fridge in the background. That fridge magnet carries the familiar form of the lovelorn volcano - the same image, in fact, as was used on the teaser poster.
The only difference is that the short's title has been replaced with the name Uku, the volcano's actual name in the film.