By Renee Maltezou ATHENS (Reuters) - Greek workers stayed at home on Thursday to protest austerity measures, in the biggest domestic challenge to Alexis Tsipras' government since he was elected in September on a promise to cushion the impact of years of economic hardship. Many flights were grounded, hospitals ran on skeleton staff, ships were docked at port and public offices stayed shut across the country in the first nationwide walkout called by Greece's largest private and public sector unions in a year. Tsipras came to power in January promising to end the austerity imposed by Greece's international lenders, then accepted the unpopular terms of a third bailout when faced with the prospect of an exit from the euro zone.