Day 35 – Multi-Stage Builds & Docker Hub

Task

Today's goal is to build optimized images and share them with the world.

Multi-stage builds are how real teams ship small, secure images. Docker Hub is how you distribute them. Both are interview favourites.

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Expected Output

• A markdown file: day-35-multistage-hub.md
• Dockerfiles demonstrating multi-stage builds
• An image pushed to your Docker Hub account

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Challenge Tasks

Task 1: The Problem with Large Images

1. Write a simple Go, Java, or Node.js app (even a "Hello World" is fine)
2. Create a Dockerfile that builds and runs it in a single stage
3. Build the image and check its size

Note down the size — you'll compare it later.

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Task 2: Multi-Stage Build

1. Rewrite the Dockerfile using multi-stage build:
   • Stage 1: Build the app (install dependencies, compile)
   • Stage 2: Copy only the built artifact into a minimal base image (alpine, distroless, or scratch)
2. Build the image and check its size again
3. Compare the two sizes

Write in your notes: Why is the multi-stage image so much smaller?

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Task 3: Push to Docker Hub

1. Create a free account on Docker Hub (if you don't have one)
2. Log in from your terminal
3. Tag your image properly: yourusername/image-name:tag
4. Push it to Docker Hub
5. Pull it on a different machine (or after removing locally) to verify

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Task 4: Docker Hub Repository

1. Go to Docker Hub and check your pushed image
2. Add a description to the repository
3. Explore the tags tab — understand how versioning works
4. Pull a specific tag vs latest — what happens?

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Task 5: Image Best Practices

Apply these to one of your images and rebuild:

1. Use a minimal base image (alpine vs ubuntu — compare sizes)
2. Don't run as root — add a non-root USER in your Dockerfile
3. Combine RUN commands to reduce layers
4. Use specific tags for base images (not latest)

Check the size before and after.

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Hints

• Multi-stage: use FROM ... AS builder then COPY --from=builder
• Login: docker login
• Tag: docker tag local-image:tag username/repo:tag
• Push: docker push username/repo:tag
• Non-root user: RUN adduser + USER

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Submission

1. Add your Dockerfiles and day-35-multistage-hub.md to 2026/day-35/
2. Include the link to your Docker Hub repo
3. Commit and push to your fork

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Learn in Public

Share your before/after image sizes on LinkedIn — the difference is always impressive.

#90DaysOfDevOps #DevOpsKaJosh #TrainWithShubham

Happy Learning! TrainWithShubham