Copying from build stage to final stage in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using multiple stages in a Dockerfile?

Multiple stages help separate the build environment from the final runtime environment. This keeps the final image small and secure by copying only needed files from the build stage.

beginner

How do you copy files from a build stage to the final stage in a Dockerfile?

Use the COPY --from=<stage-name> <source> <destination> command to copy files from a named build stage to the current stage.

beginner

What does the --from flag do in the COPY command?

The --from flag tells Docker to copy files from a previous build stage instead of the local build context.

intermediate

Why is it beneficial to copy only specific files from the build stage to the final stage?

Copying only specific files reduces the final image size, improves security by excluding build tools, and speeds up deployment.

intermediate

Give an example of a simple multi-stage Dockerfile copying a binary from build to final stage.

Example:

FROM golang:1.20 AS build
WORKDIR /app
COPY . .
RUN go build -o myapp

FROM alpine:latest
COPY --from=build /app/myapp /usr/local/bin/myapp
CMD ["/usr/local/bin/myapp"]

What does the --from option in the COPY command specify?

A The destination directory inside the container

B The network mode for the build

C The user to run the copy command

D The source build stage to copy files from

The --from option specifies which build stage to copy files from in a multi-stage Dockerfile.

Why use multi-stage builds in Docker?

A To reduce final image size by copying only needed files

B To run multiple containers at once

C To speed up network downloads

D To automatically update images

Multi-stage builds allow copying only necessary files to the final image, reducing size and improving security.

In a multi-stage Dockerfile, what happens if you omit the --from flag in COPY?

A Docker copies files from the internet

B Docker copies files from the local build context

C Docker throws an error

D Docker copies files from the last build stage

Without --from, COPY copies files from the local build context (your project folder).

Which command copies a file named app from a build stage named builder to the final image?

A COPY /app --from=builder /app

B COPY app builder:/app

C COPY --from=builder /app /app

D COPY builder app /app

The correct syntax is COPY --from=builder /app /app to copy from the build stage named 'builder'.

What is a key benefit of separating build and final stages in Docker?

A Smaller, cleaner final images

B Faster internet speed

C More CPU cores used

D Automatic code testing

Separating build and final stages keeps the final image small and free of unnecessary build tools.

Explain how to copy files from a build stage to the final stage in a Dockerfile and why this is useful.

Think about separating build environment from runtime environment.

Describe a simple example of a multi-stage Dockerfile that builds a binary and copies it to a smaller final image.

Use a language like Go or C for the example.