Related to this issue in CRA Can change REACT_APP_... env vars in docker-compose.yml and they will be inserted at container start.
It respects anything starting with REACT_APP_ in both .env and docker runtime environment
docker-compose up
<script type="text/javascript" src="config.js"></script>
This is added in html head to ensure window object is populated before any other javascript runs.
env2js.js is run everytime before the server starts First it reads the .env content into process.env without replacing anything already there It then reformats all REACT_APP variables into javascript, and writes to a config.js in the build folder.