What is Docker? A Beginner-Friendly Guide to Containers

🐳 What is Docker?

Docker is a tool that helps you package your application and all its dependencies into a single unit called a container.
This container runs exactly the same on any machine — laptop, server, or cloud — solving the “but it worked on my system!” problem.

🧱 Think of it like this:

🧁 Your application is like a cake.
🍫 It needs ingredients (Python, Java, Node, libraries, etc.)
🥡 Docker puts everything needed (cake + ingredients) into a container box.
📦 You can now run this container anywhere, and it will work the same.

🧰 Why Docker is Useful:

✅ Ensures consistency across environments (dev, test, prod)
⚡ Fast setup — no need to install dependencies manually
📦 Lightweight compared to virtual machines
🚀 Great for DevOps: works well with CI/CD tools like Jenkins, GitHub Actions, etc.

📂 Key Concepts:

Term	Meaning
Image	A blueprint (like a recipe) of a container
Container	A running instance of an image
Dockerfile	A script with steps to build an image
Docker Hub	Online registry of pre-built images (like GitHub for Docker)

⚙️ Basic Docker Commands:

docker --version  # Check Docker installed
docker pull nginx             # Download image from Docker Hub
docker run nginx              # Run container from image
docker ps                     # List running containers
docker stop <container_id>    # Stop a container
docker build -t myapp . 
# # Use official Java image
FROM openjdk:17
COPY . /app
WORKDIR /app
RUN javac MyApp.java
CMD ["java", "MyApp"]

Then run:
docker build -t my-java-app .
docker run my-java-app

Docker

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