Few protagonists are more compelling than the underdog sports hero, and “Strider” adds a new icon to the cinematic pantheon, upholding the grand tradition of “Rudy,” “Hoosiers,” and “Million Dollar Baby.” Jody (Yelena Friedman), a lonely teenage girl, joins forces with Hannah (Maggie Alexander), a disgraced track coach, and together they spend a summer training for the Pinnacle Games, the biggest track-and-field competition on the East Coast.