Expand and Improve docs/docker_guide.md

[RickyDicaprio]

The current Docker guide is either missing or incomplete. Contributors must easily understand how to run the project using Docker.

Tasks:

1. Add a clear explanation of what Docker is
2. Add installation steps for Windows, Linux, and macOS
3. Explain the structure of the Dockerfile in the repo
4. Write commands for:
   • docker build
   • docker run
   • docker ps
   • docker stop
   • docker exec -it bash
5. Add a section for common errors & fixes
6. Add an example workflow: “How to run this project in Docker”
7. Add a diagram/image explaining the Docker workflow
8. Link the Docker guide inside README.md

Goal:
Make the repository beginner-friendly for people who want to run the project in a Docker container.