Spot rates for shipping containers have been rising for four weeks. The latest data from the Drewry World Container Index composite shows the most significant weekly gain in the index in more than two years. The 23-month slump in ocean-freight costs appears to be ending. The Drewry World Container Index jumped 11.79% to $1,761 for a 40-foot container, the largest weekly gain since June 24, 2021 -- or the period when shipping costs worldwide were sky-high because of snarled supply chains. All major shipping lines have experienced a multi-year…