The world’s biggest makers of shipping containers are scrambling to meet a surge in demand for the metal boxes that shuttle some 90% of the goods around the global economy. A trade boom in the second half of last year caught the container producers — mostly Chinese companies — by surprise as the pandemic threw the existing supply of about 25 million boxes off their normal routes. The manufacturers have been ramping up output ever since, but they’re unable to alleviate shortages that have underpinned soaring freight rates for six months. The problem resembles those in the car industry, where auto makers cut computer chip orders early in 2020 expecting a slump in sales, only to see household consumption prove resilient. The container tightness and port congestion that’s accompanying it may extend into the second half of 2021, with recoveries in the U.S. and European economies expected to keep demand high for Chinese goods.