🚀 Day 13 of 30 Days DevOps Interview Preparation: Deploy Docker App via CI/CD

Series: 30 Days DevOps Interview Preparation

Author: Tathagat Gaikwad

In the DevOps world, one of the most common real-world tasks is packaging applications using Docker and automating deployments with CI/CD pipelines. Mastering this concept not only makes you job-ready but also helps you answer many DevOps interview questions with confidence.

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🔹 Theoretical Foundations

1. What is Docker in DevOps?

Docker is a containerization platform that allows you to package an application with all its dependencies into a lightweight container. This ensures the app runs the same way regardless of where it’s deployed – developer laptop, testing server, or cloud.

2. Why CI/CD with Docker?

CI/CD (Continuous Integration and Continuous Deployment/Delivery) automates the building, testing, and deployment of code. When combined with Docker, it ensures:

• ✅ Consistency across environments

• ✅ Faster deployments with pre-built images

• ✅ Rollback capabilities (by reverting to previous images)

• ✅ Scalability when deployed on Kubernetes, ECS, or Swarm

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🔹 Typical CI/CD Pipeline for a Dockerized App

Here’s the high-level flow of deploying a Docker app with CI/CD:

1. Code Commit → Developer pushes changes to GitHub/GitLab.

2. Pipeline Triggered → Jenkins / GitHub Actions / GitLab CI detects changes.

3. Build Stage → Application is containerized using a Dockerfile.

4. Test Stage → Run automated tests inside the container.

5. Push Stage → Push Docker image to container registry (Docker Hub, AWS ECR).

6. Deploy Stage → Deploy the containerized app to staging or production.

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🔹 Practical Example – Deploying Docker App with GitHub Actions

Let’s walk through an example where we deploy a simple Node.js Docker app via GitHub Actions.

Step 1: Dockerfile

# Use Node.js base image
FROM node:18-alpine

# Set working directory
WORKDIR /app

# Copy package.json and install dependencies
COPY package*.json ./
RUN npm install

# Copy source code
COPY . .

# Expose port
EXPOSE 3000

# Run the app
CMD ["npm", "start"]

Step 2: GitHub Actions Workflow (.github/workflows/docker-ci.yml)

name: CI/CD Pipeline for Docker App

on:
  push:
    branches: [ "main" ]

jobs:
  build:
    runs-on: ubuntu-latest

    steps:
      - name: Checkout repository
        uses: actions/checkout@v3

      - name: Set up Docker
        uses: docker/setup-buildx-action@v2

      - name: Log in to Docker Hub
        uses: docker/login-action@v2
        with:
          username: ${{ secrets.DOCKER_USERNAME }}
          password: ${{ secrets.DOCKER_PASSWORD }}

      - name: Build Docker Image
        run: docker build -t my-docker-user/devops-app:latest .

      - name: Push Docker Image to Docker Hub
        run: docker push my-docker-user/devops-app:latest

Step 3: Deploy to AWS EC2 (Example Command)

On the EC2 instance, pull the latest image and run:

docker pull my-docker-user/devops-app:latest
docker stop devops-app || true
docker rm devops-app || true
docker run -d -p 80:3000 --name devops-app my-docker-user/devops-app:latest

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🔹 Interview Preparation – Detailed Q&A

Q1. Why do we use Docker in CI/CD pipelines?
👉 To ensure the application runs consistently across environments, with reduced dependency issues. It makes testing, building, and deployment reproducible.

Q2. Difference between CI and CD?

• CI (Continuous Integration): Merges code changes, builds Docker images, and runs automated tests.

• CD (Continuous Deployment/Delivery): Pushes tested images to production automatically (or with approval).

Q3. How do you optimize Docker builds in CI/CD?
👉 Use multi-stage builds, leverage caching, use smaller base images (e.g., alpine), and .dockerignore to reduce context size.

Q4. How do you handle secrets when deploying Docker apps?
👉 Store them in a secure system like AWS Secrets Manager, HashiCorp Vault, or CI/CD tool’s secret store (never hardcode in Dockerfiles).

Q5. What deployment strategies can you use with Docker apps?
👉 Blue-Green Deployment, Canary Releases, and Rolling Updates to ensure zero-downtime deployments.

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🔹 Key Takeaways

• Docker + CI/CD = speed, consistency, and scalability.

• Automating builds and deployments reduces human error and increases reliability.

• Be interview-ready with strong knowledge of pipelines, registries, and deployment strategies.

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💡 What CI/CD tool do you prefer for Docker deployments — Jenkins, GitHub Actions, or GitLab CI? Share your thoughts!

#DevOps #Docker #CICD #AWS #DevOpsInterviewPreparation #Hashnode