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.