Building images in Compose in Docker - Step-by-Step Execution

Process Flow - Building images in Compose

Write Dockerfile

↓

Define service in docker-compose.yml with build context

↓

Run 'docker compose build'

↓

Docker reads Dockerfile

↓

Docker builds image step-by-step

↓

Image created and tagged

↓

Ready to run containers with new image

This flow shows how Docker Compose builds an image from a Dockerfile by reading the build context and creating the image step-by-step before running containers.

Execution Sample

Docker

version: '3.8'
services:
  web:
    build: ./app
    image: myapp:latest

This Compose file defines a service 'web' that builds an image from the './app' directory and tags it as 'myapp:latest'.

Process Table

Step	Action	Details	Result
1	Read docker-compose.yml	Find 'web' service with build context './app'	Prepare build context './app'
2	Start build	Run 'docker compose build web'	Docker starts reading Dockerfile in './app'
3	Parse Dockerfile	Read instructions like FROM, COPY, RUN	Prepare image layers
4	Execute FROM	Base image pulled or found locally	Base image ready
5	Execute COPY	Copy files from './app' to image	Files added to image layer
6	Execute RUN	Run commands inside image	Commands executed, changes saved
7	Complete build	All instructions processed	Image 'myapp:latest' created
8	Tag image	Assign tag 'myapp:latest'	Image tagged and ready
9	Exit	Build finished successfully	Ready to run containers with new image

💡 Build completes after processing all Dockerfile instructions and tagging the image.

Status Tracker

Variable	Start	After Step 4	After Step 6	Final
Image Layers	empty	base image layer added	files and commands layers added	complete image with all layers
Image Tag	none	none	none	myapp:latest

Key Moments - 3 Insights

Why does Docker need a build context folder?

Docker uses the build context folder (like './app') to access files needed during the build, such as those copied into the image. Without it, COPY commands would fail (see execution_table step 5).

What happens if the base image is not found locally?

Docker pulls the base image from the registry during the FROM instruction (step 4), so the build can continue with the correct starting point.

Why is the image tagged after build?

Tagging (step 8) gives the image a name to identify it easily when running containers or pushing to a registry.

Visual Quiz - 3 Questions

Look at the execution table, at which step are files copied into the image?

A Step 5

B Step 3

C Step 6

D Step 7

💡 Hint

Check the 'Action' and 'Details' columns for the COPY instruction in the execution_table.

According to variable_tracker, what is the state of 'Image Layers' after step 6?

A empty

B files and commands layers added

C base image layer added

D complete image with all layers

💡 Hint

Look at the 'Image Layers' row and the 'After Step 6' column in variable_tracker.

If the build context path is wrong, what will likely happen during the build?

A Docker will skip the build

B Docker will build successfully with empty files

C Docker will fail at the COPY step

D Docker will tag the image anyway

💡 Hint

Refer to key_moments about the importance of build context and execution_table step 5.

Concept Snapshot

Docker Compose builds images by reading a Dockerfile in a specified build context folder.
The 'build' key in docker-compose.yml points to this folder.
Running 'docker compose build' executes Dockerfile instructions step-by-step.
The final image is tagged as specified and ready to run containers.
Build context must include all files needed for COPY commands.
Base images are pulled if not found locally.

Full Transcript

This visual execution shows how Docker Compose builds images. First, you write a Dockerfile and define a service in docker-compose.yml with a build context folder. When you run 'docker compose build', Docker reads the Dockerfile instructions one by one. It starts by pulling or using a base image, then copies files from the build context, runs commands inside the image, and finally creates and tags the image. Variables like image layers grow as instructions run. Key points include the need for a correct build context folder and tagging the image after build. The execution table and variable tracker help visualize each step clearly.