Multi-stage for different environments in Docker - Step-by-Step Execution

Process Flow - Multi-stage for different environments

Start: Base Stage

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Build Stage: Compile/Build

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Test Stage: Run Tests

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Production Stage: Final Image

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Select Stage Based on Environment

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Dev Image

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Prod Image

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Test Image

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End

The Dockerfile builds multiple stages for dev, test, and prod environments, then selects the final image based on the environment.

Execution Sample

Docker

FROM node:18 AS base
WORKDIR /app
COPY package.json ./
RUN npm install

FROM base AS build
COPY . ./
RUN npm run build

FROM base AS test
COPY . ./
RUN npm test

FROM base AS prod
COPY --from=build /app/dist ./dist
CMD ["node", "dist/server.js"]

This Dockerfile creates separate stages for building, testing, and production, reusing the base stage.

Process Table

Step	Stage	Action	Files/State	Result
1	base	Set working directory and copy package.json	/app/package.json	package.json copied
2	base	Run npm install	/app/node_modules	Dependencies installed
3	build	Copy all source files	/app/src + others	Source files copied
4	build	Run npm run build	/app/dist	Build output created
5	test	Copy all source files	/app/src + others	Source files copied
6	test	Run npm test	Test results	Tests passed
7	prod	Copy build output from build stage	/app/dist	Production files ready
8	prod	Set CMD to start server	CMD set	Image ready for production

💡 All stages completed; final image chosen based on environment variable.

Status Tracker

Variable	base	build	test	prod
Working Directory	/app	/app	/app	/app
package.json	copied	inherited	inherited	inherited
node_modules	installed	inherited	inherited	inherited
Source Files	none	copied	copied	none
Build Output (dist)	none	created	none	copied from build
Test Results	none	none	created	none
CMD	none	none	none	set to start server

Key Moments - 3 Insights

Why do we copy source files again in build and test stages if base already has dependencies?

Base stage only installs dependencies; source files must be copied separately in build and test stages to run build and tests. See execution_table rows 3 and 5.

How does the prod stage get the build output without rebuilding?

Prod stage copies the build output from the build stage using 'COPY --from=build'. This avoids rebuilding. See execution_table row 7.

What decides which stage becomes the final image?

The final image is selected by specifying the target stage during docker build (e.g., --target prod). This is why all stages are built but only one is used.

Visual Quiz - 3 Questions

Look at the execution table, at which step are the tests run?

A Step 6

B Step 4

C Step 7

D Step 2

💡 Hint

Check the 'Action' column for 'Run npm test' in the execution_table.

According to variable_tracker, which stage has the 'CMD' set to start the server?

A test

B build

C prod

D base

💡 Hint

Look at the 'CMD' row in variable_tracker for the 'prod' column.

If we skip copying source files in the build stage, what will happen?

A Build output will be created normally

B Build stage will fail or produce empty output

C Tests will pass anyway

D Prod stage will copy source files automatically

💡 Hint

Refer to execution_table rows 3 and 4 where source files are copied before build.

Concept Snapshot

Multi-stage Dockerfiles let you create separate build steps for dev, test, and prod.
Use 'FROM ... AS stage' to name stages.
Copy files and run commands per stage.
Use 'COPY --from=stage' to reuse outputs.
Select final image with 'docker build --target'.
This keeps images small and environment-specific.

Full Transcript

This visual execution shows how a multi-stage Dockerfile builds different environments: base, build, test, and prod. The base stage sets up dependencies. Build and test stages copy source files and run build or tests. The prod stage copies the build output from build and sets the command to start the server. Variables like working directory, files, and commands change per stage. The final image is chosen by specifying the target stage during build. This approach keeps images efficient and environment-specific.