[WIP]: how to pass API_URL to the already built container

Topic: support for runtime environment, e.g., SITE_URL and such

The issue is that Webpack has already packed everything,
meaning that there is no way to use API_URL as I used it.

Links:
https://12factor.net/
https://github.com/mars/create-react-app-buildpack#compile-time-vs-runtime
vuejs/vue-cli#1217
facebook/create-react-app#578
https://stackoverflow.com/questions/45111936/passing-environment-variable-to-webpack-2-after-build