We propose Candoia, a novel platform and ecosystem for building and sharing Mining Software Repositories (MSR) tools. Using Candoia, MSR tools are built as apps, and the Candoia ecosystem, acting as an appstore, allows effective sharing. Candoia provides data extraction tools for curating custom datasets for user projects as well as data abstractions for enabling uniform access to MSR artifacts from disparate sources, which makes apps portable and adoptable across diverse software project settings of MSR researchers and practitioners. The structured design of a Candoia app and the languages selected for building various components of a Candoia app promote easy customization. To evaluate Candoia we have built over two dozen MSR apps for analyzing bugs, software evolution, project management aspects, and source code and programming practices, showing the applicability of the platform for building a variety of MSR apps. For testing portability of apps across diverse project settings, we tested the apps using ten popular project repositories, such as Apache Tomcat, JUnit, and Node.js and found that apps required no changes to be portable. We performed a user study to test customizability and we found that five of eight Candoia users found it very easy to customize an existing app. Candoia is available for download at www.candoia.org.