What is Docker? What are the core concepts and common commands of Docker?

Answer

Docker is an open-source containerization platform that packages applications and their dependencies into a lightweight, portable container, enabling rapid deployment and execution of applications in any environment.

Core Concepts of Docker

1. Image

A Docker image is a read-only template that contains everything needed to run an application: code, runtime, libraries, environment variables, and configuration files. Images are built in layers, with each layer being read-only.

Characteristics:

• Read-only template
• Layered structure
• Reusable and shareable
• Defined by Dockerfile

2. Container

A container is a running instance of an image. It's a lightweight, standalone executable software package that includes everything needed to run an application. Containers share the host operating system kernel but are isolated from each other.

Characteristics:

• Lightweight (compared to virtual machines)
• Fast startup (seconds)
• Resource isolation
• High portability

3. Registry

Docker registries are used to store and distribute Docker images. The most commonly used is Docker Hub, but private registries can also be set up.

Common Registries:

• Docker Hub (official public registry)
• Docker Registry (private registry)
• Harbor (enterprise private registry)
• AWS ECR, Google GCR (cloud provider registries)

Docker vs Virtual Machines

Feature	Docker Containers	Virtual Machines
Startup Speed	Seconds	Minutes
Resource Usage	MB level	GB level
Performance	Near native	Some overhead
Isolation	Process-level	Hardware-level
Portability	High	Medium
Management Complexity	Low	High

Common Dockerfile Instructions

dockerfile
# Base image
FROM ubuntu:20.04

# Maintainer information
MAINTAINER yourname@example.com

# Set working directory
WORKDIR /app

# Copy files
COPY . /app

# Install dependencies
RUN apt-get update && apt-get install -y python3

# Set environment variables
ENV PYTHONUNBUFFERED=1

# Expose port
EXPOSE 8080

# Run command
CMD ["python3", "app.py"]

Common Instructions:

• FROM: Specify base image
• RUN: Execute commands
• COPY/ADD: Copy files to image
• CMD/ENTRYPOINT: Command to run when container starts
• ENV: Set environment variables
• EXPOSE: Declare ports the container listens on
• VOLUME: Create mount points
• WORKDIR: Set working directory

Common Docker Commands

Image Operations

bash
# Search for images
docker search nginx

# Pull an image
docker pull nginx:latest

# View local images
docker images

# Delete an image
docker rmi nginx:latest

# Build an image
docker build -t myapp:v1 .

Container Operations

bash
# Run a container
docker run -d -p 80:80 --name mynginx nginx

# View running containers
docker ps

# View all containers
docker ps -a

# Stop a container
docker stop mynginx

# Start a container
docker start mynginx

# Delete a container
docker rm mynginx

# View container logs
docker logs mynginx

# Enter a container
docker exec -it mynginx /bin/bash

Advantages of Docker

1. Consistency: Development, testing, and production environments are identical
2. Portability: Build once, run anywhere
3. Rapid Deployment: Second-level startup, fast scaling
4. Resource Efficiency: Uses fewer resources compared to virtual machines
5. Microservices Architecture: Naturally supports microservice deployment
6. Version Control: Images can be versioned
7. Continuous Integration: Easy to integrate into CI/CD pipelines

Docker Best Practices

1. Use Official Base Images: Prioritize official images for security
2. Minimize Image Size: Use lightweight base images like alpine
3. Multi-stage Builds: Reduce final image size
4. Don't Store Data in Containers: Use Volumes for data persistence
5. Use .dockerignore: Exclude unnecessary files
6. One Process Per Container: Follow single responsibility principle
7. Security Scanning: Regularly scan images for vulnerabilities
8. Tag Management: Use semantic versioning for tags

Docker Network Modes

• bridge: Default mode, containers communicate through Docker bridge
• host: Container uses host network stack
• none: Container has no network interfaces
• container: Container shares another container's network stack
• custom network: Create user-defined networks

Docker Data Persistence

bash
# Create a volume
docker volume create mydata

# Mount a volume
docker run -v mydata:/data nginx

# Mount host directory
docker run -v /host/path:/container/path nginx

Docker is the foundation of modern cloud-native applications. Through containerization technology, it greatly simplifies application deployment and management, making it an indispensable and important part of the DevOps toolchain.