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Sandstorm
Sandstorm is a self-hostable web productivity suite. It's implemented as a security-hardened web app package manager.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
You've just described Sandstorm. Sandstorm runs instances (called grains) of applications in a sandbox. The application gets a limited file system and a Cap'n Proto RPC connection to the outside world over a unix socket. Sandstorm runs the HTTPS server, which turns requests into RPC calls. Sandstorm authenticates the users and provides temporary subdomains and frames to sandbox the HTML side of it.
https://sandstorm.io/