Copying from build stage to final stage in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to copy files from the build stage to the final stage.

Docker

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

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

Astage1

Bfinal

Cbuilder

Dsource

Attempts:

3 left

💡 Hint

Common Mistakes

Using the final stage name instead of the build stage.

Using an undefined stage name.

The --from=builder option tells Docker to copy files from the build stage named 'builder'.

2fill in blank

medium

Complete the Dockerfile to name the build stage correctly.

Docker

FROM node:18 AS [1]

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

Adeploy

Bruntime

Cfinal

Dbuilder

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' or 'runtime' which are usually for the last stage.

The build stage is named 'builder' to be referenced later for copying files.

3fill in blank

hard

Fix the error in the COPY command to correctly copy from the build stage.

Docker

COPY --from=[1] /usr/src/app/dist /app

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

Afinal

Bbuilder

Csource

Dlatest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' or 'latest' which are not build stage names.

The correct stage to copy from is 'builder', which is the build stage.

4fill in blank

hard

Fill both blanks to complete the multi-stage Dockerfile snippet.

Docker

FROM python:3.12 AS [1]
RUN pip install -r requirements.txt
FROM alpine:latest AS [2]
COPY --from=[1] /app /app

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

Abuilder

Bfinal

Cruntime

Dbuild

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up stage names or using undefined names.

The first stage is named 'builder' where dependencies are installed. The second stage is 'final' where files are copied.

5fill in blank

hard

Fill all three blanks to complete the Dockerfile for copying build output to final stage.

Docker

FROM golang:1.20 AS [1]
WORKDIR /src
RUN go build -o app .
FROM scratch AS [2]
COPY --from=[1] [3] /app

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

Abuilder

Bfinal

C/src/app

D/app

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong stage names or wrong source path for COPY.

The build stage is named 'builder'. The final stage is 'final'. The built binary is at '/src/app' in the build stage.

Practice

1. What is the main purpose of using COPY --from= in a multi-stage Docker build?

easy

A. To copy files from the host machine to the build stage

B. To copy files from a previous build stage to the final image

C. To copy files from the final image back to the build stage

D. To copy files between two running containers

Solution

1. Step 1: Understand multi-stage builds

   Multi-stage builds allow you to create intermediate stages for building your app.

2. Step 2: Purpose of COPY --from=

   The COPY --from= command copies files from these intermediate stages into the final image, keeping it clean and small.

3. Final Answer:

   To copy files from a previous build stage to the final image -> Option B

4. Quick Check:

   COPY --from= copies from build stage [OK]

Hint: COPY --from= copies from build stage, not host or containers [OK]

Common Mistakes:

• Confusing COPY --from= with copying from host
• Thinking it copies files between containers
• Assuming it copies files back to build stage

2. Which of the following is the correct syntax to copy the file /app/build/output.bin from a build stage named builder to the final image's /app directory?

easy

A. COPY --from=builder /app/build/output.bin /app

B. COPY /app/build/output.bin /app --from=builder

C. COPY builder:/app/build/output.bin /app

D. COPY /app/build/output.bin builder:/app

Solution

1. Step 1: Recall COPY --from= syntax

   The correct syntax is COPY --from=stage source_path destination_path.

2. Step 2: Match the paths and stage name

   COPY --from=builder /app/build/output.bin /app correctly uses --from=builder followed by source and destination paths.

3. Final Answer:

   COPY --from=builder /app/build/output.bin /app -> Option A

4. Quick Check:

   Correct COPY --from= syntax is COPY --from=stage src dst [OK]

Hint: COPY --from= comes before source and destination paths [OK]

Common Mistakes:

• Placing --from= after source and destination
• Using colon syntax like builder:/path (invalid)
• Swapping source and destination paths

3. Given this Dockerfile snippet, what files will be present in the final image's /app directory?

FROM node:18 AS builder
WORKDIR /app
RUN echo "build file" > build.txt

FROM alpine
COPY --from=builder /app/build.txt /app/

medium

A. No /app directory created

B. An empty /app directory

C. The file build.txt with content "build file"

D. The file build.txt but empty

Solution

1. Step 1: Check build stage output

   The builder stage creates /app/build.txt with content "build file".

2. Step 2: COPY from builder to final image

   The final stage copies /app/build.txt from builder to /app/ in the final image.

3. Final Answer:

   The file build.txt with content "build file" -> Option C

4. Quick Check:

   File copied with content preserved [OK]

Hint: COPY --from= copies actual files, not empty placeholders [OK]

Common Mistakes:

• Assuming directory is empty after copy
• Thinking file content is lost
• Assuming /app directory is missing

4. Identify the error in this Dockerfile snippet that tries to copy from a build stage named builder:

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

FROM alpine
COPY /app/myapp --from=builder /usr/local/bin/myapp

medium

A. The --from=builder flag is placed after the source path

B. The build stage name builder is missing

C. The destination path /usr/local/bin/myapp is invalid

D. The RUN go build command is incorrect

Solution

1. Step 1: Check COPY command syntax

   The correct syntax is COPY --from=builder source destination. The flag must come before paths.

2. Step 2: Identify error in given command

   The command places --from=builder after the source path, which is invalid syntax.

3. Final Answer:

   The --from=builder flag is placed after the source path -> Option A

4. Quick Check:

   --from= must come before source path [OK]

Hint: Place --from= before source path in COPY command [OK]

Common Mistakes:

• Putting --from= after source path
• Forgetting to name the build stage
• Assuming destination path is invalid without checking

5. You want to build a Docker image for a Python app. The build stage installs dependencies and creates a dist/app.tar.gz file. The final stage should only copy this archive and extract it to /app. Which Dockerfile snippet correctly copies and extracts the archive from the build stage named builder?

hard

A. FROM python:3.12 AS builder\nWORKDIR /src\nRUN pip install -r requirements.txt\nRUN tar czf dist/app.tar.gz .\n\nFROM python:3.12-slim\nWORKDIR /app\nCOPY --from=builder /src/dist/app.tar.gz /app/\nRUN tar xzf /app/app.tar.gz

B. FROM python:3.12 AS builder\nWORKDIR /src\nRUN pip install -r requirements.txt\nRUN tar czf dist/app.tar.gz .\n\nFROM python:3.12-slim\nWORKDIR /app\nCOPY /src/dist/app.tar.gz --from=builder ./\nRUN tar xzf app.tar.gz

C. FROM python:3.12 AS builder\nWORKDIR /src\nRUN pip install -r requirements.txt\nRUN tar czf dist/app.tar.gz .\n\nFROM python:3.12-slim\nWORKDIR /app\nCOPY --from=builder /dist/app.tar.gz ./\nRUN tar xzf app.tar.gz

D. FROM python:3.12 AS builder\nWORKDIR /src\nRUN pip install -r requirements.txt\nRUN tar czf dist/app.tar.gz .\n\nFROM python:3.12-slim\nWORKDIR /app\nCOPY --from=builder /src/dist/app.tar.gz ./\nRUN tar xzf app.tar.gz

Solution

1. Step 1: Verify COPY --from= syntax and source path

   The builder stage (WORKDIR /src) creates dist/app.tar.gz at /src/dist/app.tar.gz. Correctly uses COPY --from=builder /src/dist/app.tar.gz ./ with --from=builder first and absolute source path.

2. Step 2: Confirm extraction command and paths

   Final stage WORKDIR /app, copy to ./ places app.tar.gz there. RUN tar xzf app.tar.gz extracts correctly.

3. Final Answer:

   COPY --from=builder /src/dist/app.tar.gz ./\nRUN tar xzf app.tar.gz -> Option D

4. Quick Check:

   COPY --from= before source path, extract in correct directory [OK]

Hint: COPY --from= before source path; extract where file copied [OK]

Common Mistakes:

• Placing --from= after source path
• Using wrong source path like /dist/app.tar.gz instead of /src/dist/app.tar.gz
• Extracting archive from wrong directory