Both our attempts to use a dynamically generated manifest failed. It is
not clear if these failures are browsers bugs or a misuse of the
respective feature.

We’ve tried:

1. data-uri manifest
   a very straight-forward approach that injected the
   manifest as base64-encoded URI into the page.
   data-uri evaluation seems to mess with the handling
   of host relative URLs like /.
2. service-worker MITM
   Once the env was loaded we registered the service
   worker and used it’s request MITM features to
   intercept the request to the /manifest.json URL
   and returned our locally created manifest instead.
   But at least chromium still wouldn’t want to show the
   PWA install button afterwards.

The current approach hard-codes the /~local/manifest.json URL that can
be overwritten on the service side with a proper web app manifest.