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.

Practice

1. What is the main purpose of using named build stages in a Dockerfile?

easy

A. To automatically update Docker images

B. To split the build process into parts and reuse files between them

C. To create multiple Dockerfiles in one file

D. To run multiple containers at the same time

Solution

1. Step 1: Understand what named build stages do

   Named build stages let you divide your Docker build into parts with names, making it easier to manage.

2. Step 2: Recognize the benefit of reusing files

   You can copy files from one named stage to another, which helps keep images smaller and cleaner.

3. Final Answer:

   To split the build process into parts and reuse files between them -> Option B

4. Quick Check:

   Named build stages = split and reuse [OK]

Hint: Named stages let you copy files between build steps [OK]

Common Mistakes:

• Confusing named stages with running multiple containers
• Thinking named stages create multiple Dockerfiles
• Believing named stages update images automatically

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

easy

A. STAGE node:18 builder

B. BUILD node:18 AS builder

C. FROM node:18 AS builder

D. FROM node:18 WITH builder

Solution

1. Step 1: Recall Dockerfile syntax for naming stages

   The correct syntax uses the FROM instruction followed by the image and AS plus the stage name.

2. Step 2: Check each option

   Only FROM node:18 AS builder uses FROM node:18 AS builder, which is correct syntax.

3. Final Answer:

   FROM node:18 AS builder -> Option C

4. Quick Check:

   Named stage syntax = FROM ... AS ... [OK]

Hint: Use FROM image AS name to name stages [OK]

Common Mistakes:

• Using BUILD or STAGE instead of FROM
• Omitting the AS keyword
• Using WITH instead of AS

3. Given this Dockerfile snippet, what will be the final image size compared to a single-stage build?

FROM node:18 AS builder
RUN npm install
FROM node:18
COPY --from=builder /app/node_modules /app/node_modules

medium

A. Smaller, because only needed files are copied from builder

B. Larger, because it includes both stages fully

C. Same size, because both stages use the same base image

D. Build will fail due to syntax error

Solution

1. Step 1: Understand multi-stage build effect on image size

   Multi-stage builds copy only required files from earlier stages, reducing final image size.

2. Step 2: Analyze the COPY --from usage

   The final stage copies only /app/node_modules from the builder, not the whole builder image.

3. Final Answer:

   Smaller, because only needed files are copied from builder -> Option A

4. Quick Check:

   Multi-stage copy reduces image size [OK]

Hint: Copy only needed files from builder stage to reduce size [OK]

Common Mistakes:

• Assuming both stages are fully included in final image
• Thinking same base image means same size
• Confusing syntax causing build failure

4. Identify the error in this Dockerfile snippet using named build stages:

FROM python:3.12 AS build
RUN pip install -r requirements.txt
FROM python:3.12
COPY --from=builder /app /app

medium

A. The stage name 'build' is misspelled as 'builder' in COPY

B. Missing RUN command in second stage

C. Cannot use COPY between stages

D. FROM instruction must be last in Dockerfile

Solution

1. Step 1: Check stage names consistency

   The first stage is named build, but COPY uses --from=builder, which is incorrect.

2. Step 2: Understand COPY --from usage

   The --from must match the exact stage name to copy files correctly.

3. Final Answer:

   The stage name 'build' is misspelled as 'builder' in COPY -> Option A

4. Quick Check:

   Stage name mismatch causes COPY error [OK]

Hint: Match stage names exactly in COPY --from [OK]

Common Mistakes:

• Assuming COPY can't copy between stages
• Ignoring stage name spelling
• Thinking RUN is required in second stage

5. You want to build a Docker image that compiles a Go app in one stage and produces a small final image with only the binary. Which Dockerfile snippet correctly uses named build stages to achieve this?

hard

A. FROM alpine:latest
RUN apk add go
WORKDIR /app
COPY . .
RUN go build -o myapp
CMD ["myapp"]

B. FROM alpine:latest AS builder
RUN apk add go
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM golang:1.20
COPY --from=builder /app/myapp /usr/local/bin/myapp
CMD ["myapp"]

C. FROM golang:1.20
WORKDIR /app
COPY . .
RUN go build -o myapp
CMD ["myapp"]

D. FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine:latest
COPY --from=builder /app/myapp /usr/local/bin/myapp
CMD ["myapp"]

Solution

1. Step 1: Identify multi-stage build for compiling and final image

   FROM golang:1.20 AS builder
   WORKDIR /app
   COPY . .
   RUN go build -o myapp
   FROM alpine:latest
   COPY --from=builder /app/myapp /usr/local/bin/myapp
   CMD ["myapp"]

   uses a builder stage with golang image to compile, then copies only the binary to a small alpine image.

2. Step 2: Check other options for correctness

   FROM alpine:latest AS builder
   RUN apk add go
   WORKDIR /app
   COPY . .
   RUN go build -o myapp
   FROM golang:1.20
   COPY --from=builder /app/myapp /usr/local/bin/myapp
   CMD ["myapp"]

   installs Go in alpine, which is less efficient. Single-stage builds do not use multi-stage builds, so final image is large.

3. Final Answer:

   Uses golang:1.20 AS builder to compile and alpine:latest for the final binary-only image -> Option D

4. Quick Check:

   Multi-stage build with builder and final stage =

   FROM golang:1.20 AS builder
   WORKDIR /app
   COPY . .
   RUN go build -o myapp
   FROM alpine:latest
   COPY --from=builder /app/myapp /usr/local/bin/myapp
   CMD ["myapp"]

   [OK]

Hint: Use builder stage to compile, final stage to copy binary only [OK]

Common Mistakes:

• Installing build tools in final image
• Not using multi-stage build
• Copying entire source instead of just binary