PHILADELPHIA — From Maine to Texas, dockworkers at 36 ports across the eastern U.S. are now on strike for the first time in decades. And the work stoppage could snarl supply chains — leading to shortages and higher prices if it stretches on for more than a few weeks. Workers began walking picket lines early Tuesday in a strike over wages and the ports’ use of automation, though some progress had been reported in the latest contract talks.