Docker has become one of the most popular tools for building, packaging, and running applications in a lightweight, consistent, and isolated environment called containers.

💡 Virtualization vs. Containers

Traditional virtualization uses a hypervisor, which sits on top of the operating system or directly on hardware to run multiple virtual machines.
On the other hand, Docker builds a lightweight layer on top of the operating system, allowing you to run multiple isolated applications without the heavy overhead of full virtual machines.

⚡ Why use Docker?

Virtualization: Runs applications in isolated environments without affecting the host system.
Lightweight: Uses fewer resources compared to virtual machines.
Reusability: Easily reuse and share images across teams and environments.

🐳 Installing Docker on Ubuntu (Quick Steps)

sudo apt update
sudo apt install docker.io
sudo systemctl start docker
sudo systemctl enable docker

Then, add your user to the Docker group to run Docker without sudo:

sudo usermod -a -G docker $USER

✅ This command adds your current user to the docker group, allowing you to run Docker commands without root permissions.

📄 Dockerfile: The Blueprint

A Dockerfile contains instructions that define everything needed to build a container for a specific application, including its code, dependencies, and setup steps.

In short, it is used to create a Docker image, which is then used to spin up containers on the Docker engine.

Dockerfile syntax basics

FROM        # Base image
WORKDIR     # Set working directory
COPY        # Copy files into image
ENV         # Set environment variables
EXPOSE      # Expose ports
RUN         # Run commands
CMD         # Command to run when container starts

📦 Working with Images and Containers

Pull an image from Docker Hub

docker pull image_name

Build an image from a Dockerfile

docker build . -t tag/name

Run a container

docker run -d --name my-db -e MYSQL_ROOT_PASSWORD=password mysql:latest

docker run: Starts and runs a new container.
-d: Detached mode (runs in background).
--name my-db: Names the container "my-db".
-e: Sets environment variables.
MYSQL_ROOT_PASSWORD=password: Sets the MySQL root password.
mysql:latest: Uses the latest MySQL image.

🛠 Useful Docker Commands

Stop a container

docker stop container_id

Remove a container

docker rm container_id

Open a Bash shell inside a running container

docker exec -it container_id bash

This allows you to look around or run commands directly inside the container.

Run MySQL client directly in a container

sudo docker exec -it <container_id> mysql -u root -p

✅ Instead of first opening bash, this directly opens the MySQL client inside the container.

✅ Conclusion

Docker simplifies the process of packaging and running applications, making it easy to create consistent environments across development, testing, and production. By learning to work with Dockerfiles, images, and containers, you can make your applications more portable and efficient.

🚀 Understanding Docker: A Practical Guide