The underdog is a good position for Channing Tatum. Despite being one of the most beloved himbos of Hollywood, thanks to his affable screen presence, up-for-anything attitude, and obviously, his good looks, it still feels like we, as a population, underestimate Tatum a bit, especially as he makes his directorial debut with “Dog.” Tatum shares the director’s chair with frequent producing partner and “Magic Mike” and “Magic Mike XXL” writer Reid Carolin, who is also making his directorial debut. Carolin penned the script with Brett Rodriguez, about a former Army Ranger, Jackson Briggs (Tatum), w…