How do you build an open source community not around a single software or use case, but around an entire city? Since 2019 CityLAB Berlin, a public innovation lab funded by the Berlin Senate Chancellery and run by the Technologiestiftung Berlin, has been working to promote the use of open source software in government contexts. These efforts include developing open source prototypes for and with Berlin government agencies, creating resources to help both government employees as well as open source developers learn how to become more involved in open source software in the context of the city government, and creating outreach formats for engaging with both city employees and developers around open source. The goal is to turn http://github.com/berlin into a hub for open source development for the city government. In this talk, members of the CItyLAB Berlin team will discuss how they are trying to create an open source community from the ground up and present a vision for the potential of open source for our cities.