Environment variable not accessible in public/index.html #1438

My public/index.html seems to have access to PUBLIC_URL but not to any other environment variable.

$ cat .env
REACT_APP_ASSETS_URL=abc

still when referring the var REACT_APP_ASSETS_URL is not resolved in public/index.html:

npm start results in URIError: Failed to decode param '/%REACT_APP_ASSETS_URL%/main.min.js' and the html still has the var string unresolved whereas the js code can access the var with <strong>{process.env.REACT_APP_ASSETS_URL}</strong>

I think not only the PUBLIC_URL var should be accessible in the html, but all vars.

Background:
My react app is included in a page that already has some header and footer which comes with external js and css. I copied the header and footer html to the public/index.html, but like to retrieve the legacy css and js from an external server.