Targeting specific stages in Docker - Step-by-Step Execution

Process Flow - Targeting specific stages

Start Docker Build

↓

Parse Dockerfile

↓

Identify stages

↓

Check target stage specified?

No→Build all stages

Yes↓

Build only target stage

↓

Output image for target stage

↓

End Build

Docker reads the Dockerfile, finds all stages, then builds only the specified target stage if given, otherwise builds all stages.

Execution Sample

Docker

FROM alpine AS base
RUN echo base

FROM base AS final
RUN echo final

This Dockerfile has two stages: 'base' and 'final'. We can build either stage separately.

Process Table

Step	Action	Stage Targeted	Command Run	Output
1	Start build with target 'base'	base	RUN echo base	base
2	Build stage 'base' completed	base	-	-
3	Build stops after 'base' stage	base	-	-
4	Start build with target 'final'	final	RUN echo base	base
5	Continue build 'final' stage	final	RUN echo final	final
6	Build stage 'final' completed	final	-	-
7	Build stops after 'final' stage	final	-	-

💡 Build stops after completing the specified target stage.

Status Tracker

Variable	Start	After Step 1	After Step 4	Final
Current Stage	none	base	final	final
Image Built	none	base image	final image	final image

Key Moments - 2 Insights

Why does the build stop after the target stage?

Because specifying a target stage tells Docker to build only up to that stage, as shown in execution_table rows 3 and 7.

What happens if no target stage is specified?

Docker builds all stages in order, but this example focuses on targeted builds, so no target means full build.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after step 1 when targeting 'base'?

A base

B final

C none

D error

💡 Hint

Check the 'Output' column in row 1 of execution_table.

At which step does the build stop when targeting 'final'?

A Step 3

B Step 7

C Step 5

D Step 2

💡 Hint

Look for the row mentioning build stop after 'final' stage in execution_table.

If we remove the target option, how would the build behave?

A Build only 'base' stage

B Build only 'final' stage

C Build all stages sequentially

D Fail to build

💡 Hint

Refer to key_moments about what happens if no target stage is specified.

Concept Snapshot

Docker multi-stage builds allow naming stages.
Use --target <stage> to build only that stage.
Build stops after the target stage completes.
Without --target, all stages build in order.
This saves time and image size when only part is needed.

Full Transcript

This visual execution shows how Docker builds specific stages when using the --target option. The Dockerfile has two stages: base and final. When building with --target base, Docker runs commands only in the base stage and stops. When building with --target final, Docker builds base first (because final depends on it), then final, then stops. If no target is specified, Docker builds all stages. Variables track which stage is current and which image is built. Key moments clarify why the build stops after the target stage and what happens if no target is given. The quizzes test understanding of outputs at steps and build stopping points.

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