Digital platforms have radically transformed how we work, shop, and socialize. Despite their numerous benefits, they may also threaten social justice due to unforeseen or unintended consequences of specific design choices, preventing end-users from participating equitably in the digital economy. As platform owners compete by leveraging personal data, it remains unclear how digital platforms can be designed to empower end-users to control, legally own, and benefit from their data in a privacy-preserving way. Integrating design science research with heuristic theorizing, this study proposes a design theory for end-user-centric digital platforms. We derive design theory components from over nine years of data regarding the Dataswyft platform, including five meta-requirements and eight design principles. They reveal how digital platforms can be designed for social justice to address distributive and procedural social injustices by empowering end-users, both technically and legally, to protect and control their data through a containerized microservice platform infrastructure. This platform design includes multiple data protection layers, end-user-driven data collection, reconfiguration, and exchange functionalities, safeguarding mechanisms, and semi-centralized ecosystem governance structures. By evaluating an expository instantiation of the proposed design principles, we demonstrate the applicability and utility of our design theory, paving the way for data self-sovereignty and social sustainability.