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.

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