One-time starlet was forced to retire twice during careerAt first, Billy Mackay’s mobile phone is not playing ball. But a little more button pressing, a little more angling towards a window, and reception strengthens and he’s able to play the footage.A diminutive, red-haired figure, wearing a classic Rangers No 7 shirt, flickers on screen taking a corner. The whereabouts are immediately recognisable as the old Beach End at Pittodrie stadium. It’s August 1977. He is only 16- years-old.So much promise. The company he is keeping underlines the achievement of a teenager breaking into Jock Wallace’s team for this league opener against Aberdeen. Rangers were desperate to make up for the disappointment of finishing runners’ up to Celtic in both the league and Scottish Cup the previous season.Former Rangers player Billy Mackay promotes his new book inside the Ibrox changing room…