Targeting specific stages in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to name the build stage.

Docker

FROM alpine AS [1]

Drag options to blanks, or click blank then click option'

Astart

Bfinal

Cbuilder

Dbase

Attempts:

3 left

💡 Hint

Common Mistakes

Using reserved words like 'start' or 'final' without context.

The AS builder names the stage 'builder' so it can be referenced later.

2fill in blank

medium

Complete the COPY command to copy files from the 'builder' stage.

Docker

COPY --from=[1] /app/build /app

Drag options to blanks, or click blank then click option'

Abuilder

Bstart

Cbase

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using a stage name that does not exist.

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

3fill in blank

hard

Fix the error in the FROM line to correctly name the stage.

Docker

FROM node AS [1]

Drag options to blanks, or click blank then click option'

Abuild_stage

Bbuild stage

Cbuild.stage

Dbuild-stage

Attempts:

3 left

💡 Hint

Common Mistakes

Using spaces or dots in stage names.

Stage names cannot have spaces or dots. Underscores are allowed, so 'build_stage' is correct.

4fill in blank

hard

Fill both blanks to copy from the 'builder' stage and set the working directory.

Docker

COPY --from=[1] /src /app
WORKDIR [2]

Drag options to blanks, or click blank then click option'

Abuilder

B/app

C/src

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up source and destination paths.

The COPY command uses --from=builder to copy files from the 'builder' stage. The WORKDIR is set to /app to work inside that directory.

5fill in blank

hard

Fill all three blanks to define a multi-stage build with a named stage, copy from it, and set the final command.

Docker

FROM golang:1.20 AS [1]
WORKDIR /app
RUN go build -o main .

FROM alpine
COPY --from=[2] /app/main /usr/local/bin/main
CMD ["[3]"]

Drag options to blanks, or click blank then click option'

Abuilder

Cmain

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the stage and COPY source.

Incorrect CMD syntax.

The first stage is named 'builder'. The COPY command uses --from=builder to get the built binary. The CMD runs the binary named 'main'.

Practice

1. What does the --target <stage-name> option do when building a Docker image?

easy

A. It deletes the specified stage from the Dockerfile.

B. It builds only the specified stage in a multi-stage Dockerfile.

C. It runs the container from the specified stage.

D. It tags the image with the stage name.

Solution

1. Step 1: Understand multi-stage Dockerfiles

   Multi-stage Dockerfiles have multiple build stages named with AS <stage-name>.

2. Step 2: Purpose of --target

   The --target <stage-name> option tells Docker to build only up to that named stage, skipping later stages.

3. Final Answer:

   It builds only the specified stage in a multi-stage Dockerfile. -> Option B

4. Quick Check:

   --target builds specific stage [OK]

Hint: Use --target to build just one stage fast [OK]

Common Mistakes:

• Thinking --target runs containers
• Confusing --target with image tagging
• Assuming --target deletes stages

2. Which of the following is the correct syntax to name a build stage in a Dockerfile?

easy

A. FROM ubuntu WITH builder

B. FROM ubuntu TO builder

C. FROM ubuntu STAGE builder

D. FROM ubuntu AS builder

Solution

1. Step 1: Identify stage naming syntax

   Dockerfile uses FROM <image> AS <stage-name> to name stages.

2. Step 2: Check options

   Only FROM ubuntu AS builder uses AS correctly to name the stage.

3. Final Answer:

   FROM ubuntu AS builder -> Option D

4. Quick Check:

   Stage naming uses AS keyword [OK]

Hint: Remember: FROM ... AS stageName [OK]

Common Mistakes:

• Using TO or WITH instead of AS
• Missing the AS keyword
• Confusing stage naming with container commands

3. Given this Dockerfile snippet:

FROM node:18 AS build
RUN npm install
FROM nginx:alpine
COPY --from=build /app/build /usr/share/nginx/html

What happens if you run docker build --target build .?

medium

A. Fails because --target cannot be used with COPY --from.

B. Builds both stages and creates a final image with nginx.

C. Builds only the node:18 stage and stops before nginx stage.

D. Builds only the nginx stage and ignores node:18 stage.

Solution

1. Step 1: Identify stages and target

   The Dockerfile has two stages: 'build' (node:18) and default (nginx:alpine). The command targets 'build'.

2. Step 2: Effect of --target build

   Using --target build builds only the first stage and stops before the second stage.

3. Final Answer:

   Builds only the node:18 stage and stops before nginx stage. -> Option C

4. Quick Check:

   --target stops build at named stage [OK]

Hint: Target stage builds only that stage, skips later ones [OK]

Common Mistakes:

• Assuming all stages build with --target
• Thinking COPY --from breaks build
• Confusing target stage with final image

4. You have this Dockerfile snippet:

FROM python:3.12 AS base
RUN pip install flask
FROM base
RUN pip install pytest

You run docker build --target test . but get an error: "unknown stage name test". What is the likely cause?

medium

A. The stage name 'test' is not defined correctly in the Dockerfile.

B. You must build the 'base' stage first before 'test'.

C. The Docker version does not support multi-stage builds.

D. The --target option cannot be used with Python images.

Solution

1. Step 1: Check stage naming

   The Dockerfile snippet defines 'base' with AS base correctly, but the second stage FROM base lacks AS test, so 'test' is not defined.

2. Step 2: Identify error cause

   The error "unknown stage name test" means Docker did not recognize 'test' stage due to improper definition.

3. Final Answer:

   The stage name 'test' is not defined correctly in the Dockerfile. -> Option A

4. Quick Check:

   Unknown stage means wrong or missing stage name [OK]

Hint: Check stage names exactly match in Dockerfile and command [OK]

Common Mistakes:

• Assuming stages build in order automatically
• Ignoring Docker version compatibility
• Thinking --target depends on image type

5. You want to build only the 'final' stage from this Dockerfile:

FROM golang:1.20 AS builder
RUN go build -o app .
FROM alpine:latest AS final
COPY --from=builder /app /app
CMD ["/app"]

Which command correctly builds only the 'final' stage?

medium

A. docker build --target final .

B. docker build --target builder .

C. docker build --stage final .

D. docker build --name final .

Solution

1. Step 1: Identify the stage to build

   The Dockerfile has two stages: 'builder' and 'final'. We want to build 'final'.

2. Step 2: Use correct build command

   The correct syntax to build a specific stage is docker build --target <stage-name> .. So use --target final.

3. Final Answer:

   docker build --target final . -> Option A

4. Quick Check:

   --target final builds final stage [OK]

Hint: Use --target with exact stage name to build it [OK]

Common Mistakes:

• Using --stage or --name instead of --target
• Building wrong stage name
• Forgetting the dot (.) at the end