Celtic captain Callum McGregor is motivated to continue adding to the Bhoys’ domestic dominance this campaign once competitive action gets underway. This Sunday, the reigning Scottish Premiership champions will unfurl the league flag ahead of facing Kilmarnock on home soil. With Celtic yet to sign an outfield player this window, there have been grumblings of frustration from some supporters, understandably so. Nevertheless, a strong pre-season showing from the Hoops, including victories over DC United, Manchester City and Chelsea, has lifted spirits ahead of another exciting season. Now, McGre…