Certainly! Here is an example of how you can use a configuration file to dynamically change the backend URL in your dockerized React.js application.

1. Create a configuration file that contains the backend URL. You can use a JSON format like this:
``` 
{
  "apiUrl": "http://localhost:8000"
}
``` 
2. In your React.js code, use a library like axios to make API requests. You can set the base URL for axios using the backend URL from the configuration file:
``` 
import axios from 'axios';
import config from './config.json';

const api = axios.create({
  baseURL: config.apiUrl,
});

export default api;
``` 
3. Create a Bash script that generates the configuration file with the backend URL. The script reads the backend URL from an environment variable, and writes it to the configuration file. Here's an example:
``` 
#!/bin/bash

cat > config.json <<EOF
{
  "apiUrl": "$BACKEND_URL"
}
EOF

``` 
4. In your production docker-compose file, mount the configuration file as a volume in the container that runs the React.js application, and set the BACKEND_URL environment variable. Here's an example:
``` 
version: '3'

services:
  frontend:
    build: .
    volumes:
      - ./config.json:/app/src/config.json
    environment:
      - BACKEND_URL=http://backend:8000

  backend:
    build: .
    ports:
      - "8000:8000"
``` 

This docker-compose file mounts the config.json file as a volume in the frontend service, and sets the BACKEND_URL environment variable to http://backend:8000. The frontend service can then read the backend URL from the config.json file and use it as the base URL for its API requests.

I hope this helps!
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