As the lights dim and the first 3D images flicker onto the IMAX screen, we hear the recognizable voice of Chris Evans and see flashes of Marvel superheroes, including Captain America. But Evans, who plays the good Captain in the Marvel Cinematic Universe, isn’t narrating an “Avengers: Endgame” spin-off. He’s the voice of a rescue dog in the new IMAX film “Superpower Dogs,” which plays at the Denver Museum of Nature & Science through Dec.