create-runtime-env-vars.sh

#!/bin/sh
# Take REACT_APP_* env vars and make them available at runtime
# Simplified version of https://www.freecodecamp.org/news/how-to-implement-runtime-environment-variables-with-create-react-app-docker-and-nginx-7f9d42a91d70/

set -euo pipefail

# Recreate config file
rm -rf ./env-config.js
touch ./env-config.js

# Add assignment
echo "window.env_config = {" >> ./env-config.js

# Dump all REACT env vars in to a file
# Each line represents key=value pairs
tempenv="$(env | grep REACT_APP_ | sort || true)" # Stop the script failing if grep doesn't find anything
echo "$tempenv" > .env

# Read each line in .env file
while read -r line || [ -n "$line" ];
do
  # Split env variables by character `=`
  if printf '%s\n' "$line" | grep -q -e '='; then
    varname=$(printf '%s\n' "$line" | sed -e 's/=.*//')
    varvalue=$(printf '%s\n' "$line" | sed -e 's/^[^=]*=//')
    # Append configuration property to JS file
    echo "  $varname: \"$varvalue\"," >> ./env-config.js
  fi
done < .env

echo "}" >> ./env-config.js

thanks @loveboat