1. What is Docker?

Docker is a platform to develop, ship, and run applications inside lightweight containers. It ensures consistency across different environments.

2. Key Concepts

• Image → A blueprint for containers (e.g., ubuntu, nginx).
• Container → A running instance of an image.
• Dockerfile → A script to automate image creation.
• Registry → A storage for images (e.g., Docker Hub).
• Volume → Persistent storage for containers.
• Network → Communication between containers.

3. Essential Commands

🔹 Check Docker version:

docker --version

🔹 List running containers:

docker ps

🔹 List all containers (including stopped):

docker ps -a

🔹 Stop a container:

docker stop <container_id>

🔹 Start a container:

docker start <container_id>

🔹 Restart a container:

docker restart <container_id>

🔹 Remove a container:

docker rm <container_id>

🔹 Pull an image:

docker pull <image_name>

🔹 Run a container:

docker run -d --name <name> -p <host_port>:<container_port> <image>

🔹 Remove an image:

docker rmi <image_id>

🔹 Check logs:

docker logs <container_id>

🔹 Exec into a running container:

docker exec -it <container_id> bash

4. Docker Compose

• Used to manage multi-container applications using docker-compose.yml.
• Start services:

  docker-compose up -d

• Stop services:

  docker-compose down

5. Dockerfile Example

FROM python:3.9
WORKDIR /app
COPY . .
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

6. Common Issues & Fixes

✅ "Cannot connect to Docker daemon" → Start Docker:

sudo systemctl start docker

✅ "Port already in use" → Use a different port or stop conflicting service.

🧱 1. Image = Blueprint

A Docker image is like a blueprint or recipe for creating containers.

👉 How to Create (or Pull) an Image:

docker pull ubuntu

⬆️ This downloads the Ubuntu image from Docker Hub.

You can also create your own image using a Dockerfile (we’ll come to that later).

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📦 2. Container = Running App

A container is like a running machine built from the image.

👉 How to Create and Run a Container:

docker run -it ubuntu

This will:

• Use the ubuntu image
• Start an interactive shell (-it)
• Put you inside the Ubuntu container

Now you’re inside the container’s terminal! Type exit to come back out.

👉 Run a container in background:

docker run -d ubuntu sleep 9999

👉 List containers:

docker ps -a

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📸 3. Create Your Own Image

Use a Dockerfile to define an image:

🔧 Example: Dockerfile

# Use Ubuntu as base
FROM ubuntu

# Install curl
RUN apt update && apt install -y curl

# Default command
CMD ["bash"]

👉 Build your image:

docker build -t myubuntu .

Now check your image:

docker images

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💾 4. Volume = Data Storage

Volumes let you store data outside the container so it won’t be lost when the container stops.

👉 Create a volume:

docker volume create myvolume

👉 Use volume in a container:

docker run -it -v myvolume:/data ubuntu

This mounts the volume myvolume to /data inside the container.

Now any file saved in /data will persist even after the container is deleted.

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Useful Commands

docker ps -a            # List all containers
docker images           # List all images
docker volume ls        # List all volumes
docker rm <container>   # Remove container
docker rmi <image>      # Remove image
docker volume rm <vol>  # Remove volume

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