Named build stages in Docker - Step-by-Step Execution

Process Flow - Named build stages

Start Dockerfile

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Define Stage 1: base

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Build base image

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Define Stage 2: final

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Use base stage as FROM

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Build final image

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Finish Build

Dockerfile defines multiple named stages; later stages can use earlier stages by name to copy artifacts, enabling efficient multi-step builds.

Execution Sample

Docker

FROM python:3.12 AS base
RUN pip install flask

FROM base AS final
COPY . /app
CMD ["python", "/app/app.py"]

This Dockerfile builds a base image with Python and Flask, then a final image that copies app files and runs the app.

Process Table

Step	Stage	Instruction	Action	Result
1	base	FROM python:3.12 AS base	Start base stage from python:3.12	Base stage initialized
2	base	RUN pip install flask	Install Flask in base image	Flask installed in base stage
3	final	FROM base AS final	Start final stage using base stage	Final stage initialized with base as base
4	final	COPY . /app	Copy current directory to /app in final image	App files copied to /app
5	final	CMD ["python", "/app/app.py"]	Set command to run app.py	Final image ready with command set
6	-	-	-	Build finished successfully

💡 All stages built; final image ready using named stages for efficient layering

Status Tracker

Variable	Start	After Step 2	After Step 5	Final
base image	none	python:3.12 + Flask installed	same	ready
final image	none	none	base image + app files copied + CMD set	ready

Key Moments - 3 Insights

Why do we name a build stage with AS base or AS final?

Naming stages allows us to refer to them later in the Dockerfile, like in step 3 where 'final' uses 'base' as its starting point, enabling reuse and smaller images.

What happens if we don't name the first stage but try to use it later?

Without a name, you cannot refer to that stage later. The build would fail at step 3 because 'base' would be undefined.

Does the final image include all layers from the base stage?

Yes, the final stage starts FROM the base stage, so it includes all its layers plus any new instructions like copying files.

Visual Quiz - 3 Questions

Look at the execution table at step 3. What does 'FROM base AS final' do?

A Copies files from base to final automatically

B Starts a new stage named final using the base stage as its starting point

C Runs the base stage commands again inside final

D Ends the build process

💡 Hint

Check the 'Action' and 'Result' columns at step 3 in the execution table

At which step are the app files copied into the final image?

A Step 5

B Step 2

C Step 4

D Step 1

💡 Hint

Look for the COPY instruction in the execution table

If we remove 'AS base' from step 1, what will happen at step 3?

A Build will fail because 'base' stage is not defined

B Build will succeed but final stage will be empty

C Build will ignore step 3 and continue

D Build will use python:3.12 again automatically

💡 Hint

Refer to the key moment about naming stages and step 3 in the execution table

Concept Snapshot

Named build stages in Dockerfile:
- Use 'FROM image AS name' to name a stage
- Later stages can use 'FROM name' to start from that stage
- Enables multi-step builds and smaller images
- Copy artifacts between stages easily
- Final image is the last stage built

Full Transcript

This visual execution shows how Dockerfile named build stages work. First, a base stage is created from python:3.12 and Flask is installed. This stage is named 'base'. Next, a final stage starts from 'base' and copies application files into it, then sets the command to run the app. Naming stages allows reuse and efficient builds. The execution table traces each step, showing how stages build up. Variables track the state of base and final images. Key moments clarify why naming is important and what happens if omitted. The quiz tests understanding of stage naming, copying files, and build errors. The snapshot summarizes the key points for quick reference.