Borussia Dortmund undoubtedly taught Celtic their harshest lesson of the season so far with Tuesday night’s 7-1 thrashing in the Champions League. Dortmund were relentless with their deluge of goals against the Bhoys, punishing individual errors and collective underperformance on the big European stage. Celtic had won all nine of their matches prior to the trip to Germany. It was only lower-league side Falkirk that gave Brendan Rodgers’ side a fright, and the Championship side eventually succumbed to a 5-2 defeat after taking the lead twice in the Scottish League Cup tie. Their manager, John M…