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DevOps Bootcamp · Lesson

Building Docker Images with Actions

Automate the process of building Docker images for your applications directly within your GitHub Actions workflows.

Building Docker Images with Actions is a free DevOps Bootcamp lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the DevOps Bootcamp learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What Are Docker Images?

Welcome! In this lesson, we'll learn to build Docker images using GitHub Actions.

First, what's a Docker image? Think of it as a lightweight, standalone, executable package that includes everything needed to run a piece of software:

  • Your code
  • A runtime (e.g., Java, Python)
  • System tools and libraries
  • Configuration files

It's like a blueprint for creating Docker containers, ensuring your application runs consistently across different environments.

Why Automate Docker Builds?

Manually building Docker images can be repetitive and error-prone. This is where automation comes in handy!

By integrating Docker image builds into your CI/CD pipeline with GitHub Actions, you gain:

  • Consistency: Every build follows the same steps.
  • Speed: Automated builds are faster than manual ones.
  • Reliability: Reduce human errors.
  • Integration: Builds are triggered automatically with code changes.

It's a core practice in modern DevOps!

The Dockerfile Blueprint

A Dockerfile is a text file containing all the commands to assemble a Docker image. It's like a recipe.

Here's a very simple Dockerfile example:

FROM alpine:latest
WORKDIR /app
COPY . .
CMD ["echo", "Hello from Docker!"]

Understanding the Dockerfile

Let's break down that simple Dockerfile:

  • FROM alpine:latest: This specifies the base image. We're starting with a small Linux distribution called Alpine.
  • WORKDIR /app: Sets the working directory inside the image.
  • COPY . .: Copies all files from your project directory into the /app directory inside the image.
  • CMD ["echo", "Hello from Docker!"]: This is the command that will run when a container starts from this image.

This file sits in the root of your project.

Setting Up Your Workflow

To build Docker images with GitHub Actions, you'll create a YAML file (e.g., .github/workflows/docker-build.yml) in your repository.

This file defines your workflow, which is a series of automated steps.

A basic workflow for building an image usually involves:

  1. Checking out your code.
  2. Setting up the Docker build environment.
  3. Building the Docker image.

Step 1: Get Your Code

The first step in almost any GitHub Actions workflow is to get your repository's code. This is done using the actions/checkout action.

It clones your repository into the runner, making your Dockerfile and application code available for the build process.

Here's how it looks in your workflow file:

      - name: Checkout repository code
        uses: actions/checkout@v4

Step 2: Set Up Docker Buildx

Next, we need to set up the Docker build environment. The docker/setup-buildx-action is highly recommended.

Buildx is a Docker component that extends Docker's build capabilities, supporting advanced features like multi-platform builds.

Add this step to your workflow:

      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3

Step 3: Build the Docker Image

Now for the main event: building the image! We use the docker/build-push-action for this.

Key options:

  • context: .: Specifies the build context (usually your repository root).
  • push: false: For now, we'll just build without pushing to a registry. We'll cover pushing in a later lesson.
  • tags: my-app:latest: Assigns a name and tag to your image.
      - name: Build and tag Docker image
        uses: docker/build-push-action@v5
        with:
          context: .
          push: false
          tags: my-app:latest

Full Build Workflow Example

Here's the complete GitHub Actions workflow to build a Docker image. Remember, this YAML goes into .github/workflows/docker-build.yml.

This workflow will trigger on every push to your repository.

name: Build Simple Docker Image

on: [push]

jobs:
  build-image:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository code
        uses: actions/checkout@v4

      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3

      - name: Build and tag Docker image
        uses: docker/build-push-action@v5
        with:
          context: .
          push: false
          tags: my-app:latest

Quick Check

Let's test your understanding of building Docker images with GitHub Actions.

Recap: Building Docker Images

Great job! You've learned the fundamentals of automating Docker image builds with GitHub Actions.

We covered:

  • What Docker images are and why they are crucial.
  • The role of a Dockerfile as the image blueprint.
  • Setting up a basic GitHub Actions workflow.
  • Using essential actions like actions/checkout, docker/setup-buildx-action, and docker/build-push-action.

In the next lesson, we'll explore how to push these built images to a container registry!

Frequently asked questions

Is the “Building Docker Images with Actions” lesson free?

Yes — the full text of “Building Docker Images with Actions” is free to read here on the web, and the DevOps Bootcamp course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the DevOps Bootcamp course, upgrade to CoddyKit PRO.

What will I learn in “Building Docker Images with Actions”?

Automate the process of building Docker images for your applications directly within your GitHub Actions workflows. You practise DevOps Bootcamp with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start DevOps Bootcamp?

No prior experience is required. DevOps Bootcamp on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Building Docker Images with Actions” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this DevOps Bootcamp lesson?

Yes. Every DevOps Bootcamp lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Building Docker Images with Actions
  2. Pushing Images to Registries
  3. Deploying to Kubernetes with Actions
  4. Helm Charts and Kubernetes Manifests in CI/CD
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