Why Copying from build stage to final stage in Docker? - Purpose & Use Cases

The Big Idea

Discover how to make your Docker images smaller and cleaner without extra hassle!

The Scenario

Imagine you build a big project inside a container, then you want to move only the finished app files to a smaller container for running it.

Doing this by hand means copying files out, cleaning up, and making sure nothing extra gets included.

The Problem

Manually copying files is slow and easy to mess up.

You might copy too much, making the final container big and slow.

Or miss important files, causing the app to break.

The Solution

Using Docker multi-stage builds lets you copy just what you need from the build stage to the final stage automatically.

This keeps your final container small and clean without extra work.

Before vs After

✗ Before

docker build -t myapp .
docker cp container:/app/build ./build
# then create a new Dockerfile to copy ./build

✓ After

FROM builder AS build
RUN build commands
FROM base
COPY --from=build /app/build /app/

What It Enables

You can create small, efficient containers by automatically transferring only the necessary files from build to final stage.

Real Life Example

When building a web app, you compile code and then copy only the compiled files to a lightweight server image, making deployment faster and safer.

Key Takeaways

Manual copying is slow and error-prone.

Multi-stage builds automate clean file transfer.

Final containers become smaller and more secure.