LA LIBERTAD, El Salvador—In a rock concert-like atmosphere, El Salvador President Nayib Bukele announced that his government will build an oceanside “Bitcoin City” at the base of a volcano. Bukele used a gathering of Bitcoin enthusiasts Saturday night to launch his latest idea, much as he used a an earlier Bitcoin conference in Miami to announce in a video message that El Salvador would be the first country to make the cryptocurrency legal tender, A bond offering would happen in 2022 entirely in Bitcoin, Bukele said, wearing his signature backwards baseball cap. And 60 days after financing was ready, construction would begin. The city will be built near the Conchagua volcano to take advantage of geothermal energy to power both the city and Bitcoin mining—the energy-intensive solving of complex mathematical calculations day and night to verify currency transactions. The government is already running a pilot Bitcoin mining venture at another …