Docker Day 6 | Multi Stage Builds Explained | Reduce Image Size | Spring Boot Java Demo | Docker
🐳 Welcome to Day 6 of the Docker Zero to Hero series!
In this video I'll explain what Docker Multi Stage Builds are,
why they dramatically reduce your image size, how to write
a multi stage Dockerfile for a Java application and show you
the real image size difference before and after — the numbers
will surprise you!
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🎯 WHY MULTI STAGE BUILDS ARE A PRODUCTION MUST
A single stage Dockerfile for a Java application
packages everything — JDK, Maven, source code,
build cache and your actual JAR — all into one image.
The result is a bloated image that is slow to pull,
expensive to store and full of unnecessary attack surface.
Multi Stage Builds fix this by separating build time
and runtime environments — so your final production image
contains only what is absolutely needed to run your app.
This is considered a Docker best practice in every
production environment and a very commonly asked
Docker interview question.
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📌 WHAT YOU WILL LEARN IN THIS VIDEO
✅ What is Docker Multi Stage Build and why it exists
✅ Problem with traditional single stage Dockerfiles
✅ How multi stage builds separate build and runtime
✅ How to write a multi stage Dockerfile for Java
✅ How COPY --from syntax copies artifacts between stages
✅ Real image size before and after multi stage build
✅ Why build tools must not be in production image
✅ When to use multi stage builds in real projects
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🔍 WHO IS THIS VIDEO FOR
This video is perfect for you if you are:
👉 A Java developer who wants to optimize Docker images
👉 A DevOps engineer preparing for Docker interviews
👉 Someone whose Docker images are too large and slow
👉 Anyone preparing for CKA or CKAD certification exams
👉 Someone who wants to follow Docker best practices
in real production environments
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💡 HOW MULTI STAGE BUILD WORKS
🏗️ Stage 1 — Builder
→ Starts with full JDK image
→ Copies source code into container
→ Runs Maven to compile and build JAR
→ Heavy stage — has everything needed to build
→ Never makes it into the final image
📦 Stage 2 — Runtime
→ Starts fresh with minimal JRE image
→ Uses COPY --from=builder to copy only the JAR
→ No JDK, no Maven, no source code, no build cache
→ This is the final image that gets pushed and deployed
→ Dramatically smaller, cleaner and more secure
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⚙️ WHAT IS COVERED IN THE LIVE DEMO
→ Writing single stage Dockerfile for Java app
→ Building and checking bloated image size
→ Writing multi stage Dockerfile for same app
→ Using COPY --from to copy JAR between stages
→ Building multi stage image and comparing size
→ Showing dramatic size difference side by side
→ Verifying application runs correctly from smaller image
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⚠️ COMMON MISTAKES BEGINNERS MAKE
→ Copying entire source code into runtime stage
Only copy the final artifact — JAR or binary
→ Using JDK in runtime stage instead of JRE
JRE is enough to run — JDK is only needed to build
→ Not naming your builder stage
Always name stages using AS keyword for clarity
→ Forgetting COPY --from syntax
This is what connects your stages together
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🔔 SUBSCRIBE TO ayushbuildstech
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with real hands on tutorials —
Subscribe to ayushbuildstech and hit the bell icon
so you never miss a video in this series!
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👍 FOUND THIS HELPFUL?
If this video helped you understand Multi Stage Builds —
smash the Like button and subscribe for Day 7!
💬 COMMENT BELOW:
What was your Docker image size before and after
using multi stage builds?
Drop your numbers below — would love to see the difference!
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🔎 DOCKER ZERO TO HERO SERIES
→ Day 1 — Introduction to Docker
→ Day 2 — Docker Installation + Dockerfile Instructions
→ Day 3 — Dockerize Spring Boot Application
→ Day 4 — Docker Volumes Explained
→ Day 5 — Docker Networks Explained
→ Day 7 — Coming Soon
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