Docker in CI/CD Pipelines

Docker and CI/CD are natural partners. Containers provide consistent, reproducible environments for building, testing, and deploying applications. This lesson covers how to build Docker images in CI, optimise layer caching, run tests in containers, and push images to registries — with practical examples for GitHub Actions and GitLab CI.

---

Why Docker in CI/CD?

Benefit	Description
Reproducibility	Same image in CI, staging, and production
Isolation	Build and test in a clean environment every time
Speed	Layer caching reduces build times
Portability	The same pipeline works regardless of the CI platform
Artefact management	The image IS the deployable artefact

---

Building Images in CI

GitHub Actions

# .github/workflows/build.yml
name: Build and Push

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

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

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

      - name: Login to Docker Hub
        if: github.event_name != 'pull_request'
        uses: docker/login-action@v3
        with:
          username: \${{ secrets.DOCKER_USERNAME }}
          password: \${{ secrets.DOCKER_PASSWORD }}