Targeting specific stages in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of multi-stage build with specific target

Given the following Dockerfile, what will be the output of docker build --target builder .?

Docker

FROM alpine AS builder
RUN echo "Building stage" > /build.txt
FROM alpine
COPY --from=builder /build.txt /app/build.txt
CMD cat /app/build.txt

A

Step 1/2 : FROM alpine
 ---> Using cache
Step 2/2 : CMD cat /app/build.txt
 ---> Running in container
Removing intermediate container
Successfully built <image_id>

B

Step 1/2 : FROM alpine AS builder
 ---> Using cache
Step 2/2 : RUN echo "Building stage" > /build.txt
 ---> Running in container
Removing intermediate container
 ---> <image_id>
Successfully built <image_id>

C

Step 1/3 : FROM alpine
 ---> Using cache
Step 2/3 : COPY --from=builder /build.txt /app/build.txt
 ---> Running in container
Removing intermediate container
Successfully built <image_id>

D

Step 1/4 : FROM alpine AS builder
 ---> Using cache
Step 2/4 : RUN echo "Building stage" > /build.txt
 ---> Running in container
Removing intermediate container
 ---> Using cache
Successfully built <image_id>

💡 Hint

When you specify --target builder, Docker stops after building that stage.

Using --target builder builds only up to the stage named 'builder'. The output shows the steps up to that stage, including the RUN command creating /build.txt. The final image is from the builder stage only.

🧠 Conceptual

intermediate

Purpose of targeting specific stages in Docker builds

Why would a developer use the --target option to build a specific stage in a multi-stage Dockerfile?

A To build and test intermediate stages separately without building the entire image

B To build only the final image and skip intermediate stages

C To automatically push the image to a registry after building

D To reduce the size of the final image by removing unused layers

💡 Hint

Think about testing or debugging parts of the build process.

Using --target allows building up to a specific stage. This helps developers test or debug intermediate stages without building the entire image.

❓ Configuration

advanced

Correct Dockerfile stage targeting syntax

Which Dockerfile snippet correctly defines two stages named builder and final so that you can target the builder stage during build?

A

FROM node:18
RUN npm install
FROM node:18
COPY --from=builder /app /app
CMD node /app/index.js

B

FROM node:18
RUN npm install
FROM node:18 AS final
COPY --from=builder /app /app
CMD node /app/index.js

C

FROM node:18 AS builder
RUN npm install
FROM node:18 AS final
COPY --from=builder /app /app
CMD node /app/index.js

D

FROM node:18 AS builder
RUN npm install
FROM node:18
COPY --from=builder /app /app
CMD node /app/index.js

💡 Hint

Both stages must be named to use --target properly.

To target a stage, it must be named with AS <name>. Option C names both stages correctly, allowing --target builder to work.

❓ Troubleshoot

advanced

Error when targeting a non-existent stage

What error will Docker show if you run docker build --target test . but the Dockerfile has no stage named test?

A Error: failed to build: stage 'test' not found

B Error: failed to solve with frontend dockerfile.v0: no match for platform in manifest

C Error: unknown stage name test

D Error: invalid target stage specified

💡 Hint

Docker complains when the target stage name is missing in the Dockerfile.

If the target stage does not exist, Docker returns an error like 'failed to build: stage 'test' not found'.

🔀 Workflow

expert

Optimizing build time using targeted stages

You have a multi-stage Dockerfile with stages builder, tester, and final. You want to run tests only on the tester stage without building the final stage. Which command achieves this?

A docker build --target final .

B docker build --stage tester .

C docker build --only tester .

D docker build --target tester .

💡 Hint

Check the correct Docker CLI option for targeting a build stage.

The --target option specifies which stage to build up to. Using --target tester builds only up to the tester stage.