Building Custom Docker Images with Compose

Welcome to the final leg of your journey in mastering Docker with our course. This lesson focuses on the important skill of building custom Docker images using Docker Compose — essential when tailoring deployments beyond typical pre-built images. By the end of this lesson, you'll be adept at configuring Docker Compose to build Docker images, enhancing your ability to manage and deploy complex applications.

Docker Compose enhances its orchestration capabilities by facilitating the building of Docker images. It uses the build directive within a service definition to specify image construction parameters.

Here's a streamlined breakdown of this process:

1. Build Directive: The build directive in the docker-compose.yml file instructs Docker Compose on how to build the image.

2. Context and Dockerfile: Compose reads a specified context, typically the current directory, to find the Dockerfile. It executes each instruction in the Dockerfile to build the image layer by layer.

By understanding these steps, you can efficiently leverage Docker Compose to build custom Docker images tailored to your application needs.

Illustrating how Docker Compose builds images, consider a project directory named my-project that encompasses all necessary files to define and build a custom Docker image:

my-project/
    ├── docker-compose.yml
    ├── Dockerfile
    └── ... (additional application files)

• docker-compose.yml: Dictates the services and specifies instructions for image creation.
• Dockerfile: Details steps to build the Docker image.

This structure encapsulates the vital components required for Docker Compose to efficiently build and manage a custom image.

Here's an example of defining a service in the docker-compose.yml file to build a Docker image using the project structure. In this case, a web service constructs the image from the current directory, enabling the use of custom application code:

version: '3'

services:
    web:
        build: # Specify build settings for the image
            context: . # Set build context to the current directory
        ports:
            - "8080:80"

This configuration directs Docker Compose to use the current directory as the build context, meaning it will look for a Dockerfile in this directory to build the image.