Docker – Quick Notes & Guide 🐳

1️⃣ What is Docker?

Docker is an open-source platform that automates application deployment in lightweight containers.

• Containers are isolated, portable, and consistent across environments.
• Works with Linux, Windows, macOS.

2️⃣ Docker vs Virtual Machine

FeatureDockerVMOSShares host OSSeparate OSSizeLightweightHeavyBoot TimeSecondsMinutesResource UsageLowHighIsolationProcess-levelHardware-level

3️⃣ Key Docker Components

• Docker Engine – Runtime for containers
• Docker Image – Blueprint of container
• Docker Container – Running instance of image
• Docker Hub – Public repository
• Dockerfile – Instructions to build image

4️⃣ Basic Docker Commands

Installation Check

docker --version

docker info

List Images & Containers

docker images # list images

docker ps # running containers

docker ps -a # all containers

Run Container

docker run -d -p 80:80 nginx

Stop & Remove

docker stop <container_id>

docker rm <container_id>

docker rmi <image_name>

View Logs

docker logs <container_id>

5️⃣ Dockerfile Example

# Use official image

FROM python:3.11

# Set working directory

WORKDIR /app

# Copy requirements and install

COPY requirements.txt .

RUN pip install -r requirements.txt

# Copy source code

COPY . .

# Run app

CMD ["python", "app.py"]

Build & Run

docker build -t myapp .

docker run -d -p 5000:5000 myapp

6️⃣ Docker Networking

• bridge – Default, container-to-host communication
• host – Uses host network
• none – No networking
• overlay – Multi-host networking (Docker Swarm)

7️⃣ Docker Volumes (Persistent Storage)

docker volume create mydata

docker run -v mydata:/app/data myapp

8️⃣ Docker Compose (Multi-container Apps)

docker-compose.yml example:

version: '3'

services:

web:

image: nginx

ports:

- "80:80"

db:

image: mysql

environment:

MYSQL_ROOT_PASSWORD: root123

Run:

docker-compose up -d

docker-compose down

9️⃣ Docker Best Practices

• Use small base images
• Write multi-stage Dockerfiles
• Keep one process per container
• Use tags for versioning
• Secure Docker daemon

🔟 Interview Quick Questions

Q: Difference between image and container?

A: Image = blueprint, Container = running instance

Q: How to persist data in Docker?

A: Using volumes

Q: What is Docker Compose?

A: Tool to run multi-container apps

Q: How to expose a port?