When an Esri Web AppBuilder web app is configured with a portalUrl value served from HTTPS, the web app automatically redirects users to HTTPS when visited via HTTP. While this is best-practice in production, it can be a burden in development when you want to quickly run a local version of the web app. Below is a quick script written with Python standard libraries to serve a web app over HTTP. It works by serving a config.json that is modified to use HTTP rather than HTTPS. This allows you to keep config.json using the HTTPS configuration for production but serve the web app via HTTP during development.
The script should be saved alongside the config.json in the root of the web app. I would recommend running chmod a+x runserver to enable you to execute the server directly via ./runserver. Alternatively, you could install this somewhere on your system path to invoke from any directory (something like cp runserver /usr/local/bin/serve-esri-app for a unix-based system).