Named build stages in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of multi-stage Docker build with named stages

Given the Dockerfile below, what will be the output of the final image when running docker run --rm myapp?

Docker

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

FROM alpine
COPY --from=builder /message.txt /message.txt
CMD ["cat", "/message.txt"]

A

Hello from builder

B

cat: /message.txt: No such file or directory

C

Hello from alpine

D Error: COPY failed

💡 Hint

Look at how the --from=builder copies the file from the named stage.

The first stage named 'builder' creates a file with the text 'Hello from builder'. The final stage copies that file from the builder stage and outputs its content.

❓ Configuration

intermediate

Correct syntax for naming a build stage

Which of the following Dockerfile snippets correctly names a build stage as 'test'?

A

FROM node AS test
RUN npm test

B

FROM node WITH test
RUN npm test

C

FROM node NAMED test
RUN npm test

D

FROM node STAGE test
RUN npm test

💡 Hint

The keyword to name a stage is 'AS'.

The correct syntax to name a build stage is 'FROM AS '.

❓ Troubleshoot

advanced

Troubleshooting COPY from a non-existent stage

What error will Docker produce if you try to copy from a named stage that does not exist?

Docker

FROM alpine
COPY --from=builder /file.txt /file.txt

A COPY succeeded but file is empty

B Error: unknown stage name builder

C No error, file copied from base image

D Error: file /file.txt not found

💡 Hint

Check if the stage name used in COPY matches any defined stage.

Docker requires the stage name in COPY --from to exist. If it doesn't, it throws an error about unknown stage name.

🔀 Workflow

advanced

Order of stages in a multi-stage Dockerfile

In a Dockerfile with multiple named stages, which stage is used as the final image by default?

A The first stage defined in the Dockerfile

B The stage specified by the --target option only

C The stage named 'final' if it exists

D The last stage defined in the Dockerfile

💡 Hint

If no target is specified, Docker uses a default stage.

By default, Docker uses the last stage in the Dockerfile as the final image unless you specify a different target.

✅ Best Practice

expert

Best practice for reducing image size using named build stages

Which approach best reduces the final Docker image size when using named build stages?

A Use a single stage Dockerfile with all dependencies installed

B Install all build tools in the final stage to avoid copying files

C Use a heavy builder stage with all tools, then copy only necessary artifacts to a minimal final stage

D Copy all files from the builder stage to the final stage to ensure completeness

💡 Hint

Think about separating build environment from runtime environment.

Using a heavy builder stage with all build tools and copying only the needed files to a minimal final stage keeps the final image small and clean.