🐳 What is Docker?

Docker is a platform that allows you to package applications and their dependencies into a single “container” so they can run anywhere consistently – on your laptop, on a server, or in the cloud.

Think of it like a lightweight virtual machine, but faster, smaller, and more portable.

1️⃣ Key Concepts

1. Containers

• Isolated environments that run applications
• Share the host OS kernel (unlike VMs, which include a full OS)
• Lightweight, start in seconds

2. Images

• A read-only template used to create containers
• Contains application code, libraries, dependencies, and OS settings
• Example: nginx:latest image to run an Nginx web server

3. Docker Engine

• The core software that runs containers on your system

4. Docker Hub

• Cloud repository for sharing Docker images
• Public & private images available

2️⃣ Why Use Docker?

BenefitExplanationPortabilityRun the same container anywhere: dev → test → prodIsolationApps don’t interfere with each otherLightweightUses less memory & storage than traditional VMsScalabilityEasy to scale apps horizontallyVersion ControlDocker images can be versioned and rolled back

3️⃣ Docker vs Virtual Machine

FeatureDocker ContainerVirtual MachineOSShares host OS kernelIncludes guest OSSizeMBsGBsStartupSecondsMinutesIsolationProcess-levelHardware-levelResource UsageLowHigh

4️⃣ Basic Docker Commands

# Pull an image from Docker Hub

docker pull nginx

# Run a container

docker run -d -p 8080:80 nginx

# List running containers

docker ps

# Stop a container

docker stop <container_id>

# Remove a container

docker rm <container_id>

# List images

docker images

# Remove image

docker rmi <image_id>

5️⃣ Docker Workflow (Typical DevOps Use)

1. Write Dockerfile – Defines app environment
2. Build image – docker build -t myapp .
3. Run container – docker run -d myapp
4. Test & deploy – Use same container anywhere

6️⃣ Use Cases

• Microservices deployment
• CI/CD pipelines
• Cloud-native applications
• Testing applications in isolated environments
• Multi-tenant apps