Copying from build stage to final stage in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of multi-stage Docker build copying files

Given the following Dockerfile, what will be the output when running docker build .?

Docker

FROM alpine AS builder
RUN echo 'Hello from builder' > /message.txt

FROM alpine
COPY --from=builder /message.txt /final_message.txt
CMD cat /final_message.txt

A Hello from builder

B cat: /final_message.txt: No such file or directory

C Error: COPY failed

D Hello from final stage

💡 Hint

The COPY --from=builder copies files from the build stage named 'builder'.

The file /message.txt is created in the builder stage and copied to the final stage as /final_message.txt. The CMD prints its content.

❓ Configuration

intermediate

Correct COPY syntax for multi-stage build

Which COPY command correctly copies the directory /app from the build stage named builder to the final stage's /app directory?

A COPY /app builder:/app

B COPY /app /app --from=builder

C COPY builder:/app /app

D COPY --from=builder /app /app

💡 Hint

The --from flag must come immediately after COPY.

The correct syntax is COPY --from=builder source destination. Options B, C, and D are invalid syntax.

❓ Troubleshoot

advanced

Why does COPY --from fail in this Dockerfile?

You have this Dockerfile snippet: FROM node:18 AS builder WORKDIR /app RUN echo 'build files' > build.txt FROM node:18 COPY --from=build /app/build.txt /app/build.txt Why does the build fail with error: 'COPY failed: stat /var/lib/docker/tmp/...: no such file or directory'?

A The final stage must be named 'builder' to copy files

B The source path /app/build.txt does not exist in the builder stage

C The build stage is named 'builder', but COPY uses 'build' which does not exist

D COPY cannot copy files from previous stages

💡 Hint

Check the stage name after AS and the name used in --from.

The build stage is named 'builder', but COPY uses --from=build which is undefined, causing the error.

🔀 Workflow

advanced

Order of stages in multi-stage Dockerfile

In a multi-stage Dockerfile, which statement is true about copying files from a build stage to the final stage?

A The build stage must appear before the final stage in the Dockerfile

B You cannot copy files between stages

C Stages can be in any order; Docker resolves them automatically

D The final stage must appear before the build stage

💡 Hint

Think about how Docker builds stages sequentially.

Docker builds stages in order. To copy from a build stage, it must be defined before the final stage.

✅ Best Practice

expert

Optimizing multi-stage Docker builds for smaller final images

Which approach best reduces the final image size when copying artifacts from a build stage?

A Copy the entire build directory to the final stage

B Copy only the necessary files from the build stage using specific paths

C Install build tools in the final stage and build there

D Use a single-stage build with all tools installed

💡 Hint

Think about minimizing what goes into the final image.

Copying only needed files avoids bloating the final image with unnecessary build artifacts or tools.