Targeting specific stages in Docker - Mini Project: Build & Apply

Docker Multi-Stage Build: Targeting Specific Stages

📖 Scenario: You are building a Docker image for a simple web application. To keep the image size small, you want to use multi-stage builds. You will create two stages: a builder stage that compiles the app, and a final stage that only contains the compiled app.

🎯 Goal: Build a Dockerfile with two stages named builder and final. Then, target the builder stage to build the image.

📋 What You'll Learn

Create a Dockerfile with a builder stage using the node:18 image

Create a final stage using the nginx:alpine image

Copy the compiled app from builder to final

Build the Docker image targeting the builder stage

💡 Why This Matters

🌍 Real World

Multi-stage builds are used in real projects to reduce image size and separate build dependencies from runtime environments.

💼 Career

Knowing how to target specific stages in Docker builds is important for DevOps roles to optimize container images and deployment pipelines.

1

Create the builder stage

Create a Dockerfile with a builder stage that uses the node:18 image and sets the working directory to /app.

Docker

# Your code here

Hint

Use FROM node:18 AS builder to name the stage builder.

2

Add the final stage

Add a final stage that uses the nginx:alpine image and copies the /app/build directory from the builder stage to /usr/share/nginx/html.

Docker

FROM node:18 AS builder
WORKDIR /app
# Your code here

Hint

Use COPY --from=builder to copy files from the builder stage.

3

Add a build command targeting the builder stage

Write the Docker build command that builds the image named myapp-builder targeting the builder stage.

Docker

FROM node:18 AS builder
WORKDIR /app

FROM nginx:alpine AS final
COPY --from=builder /app/build /usr/share/nginx/html
# Your code here

Hint

Use docker build --target builder -t myapp-builder . to build only the builder stage.

4

Print the build command output

Run the build command docker build --target builder -t myapp-builder . and print the output showing the build stages.

Docker

FROM node:18 AS builder
WORKDIR /app

FROM nginx:alpine AS final
COPY --from=builder /app/build /usr/share/nginx/html

# Run the build command and show output
# Your code here

Hint

Simulate the build output by printing a success message.

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