04 — Build your Docker image in CI

Category: Docker in CI · Priority: High · Step: 5

Reuse the Dockerfile from Project 1 — build it on every push to catch broken images early.

🎯 What is this task?

In Project 1 you wrote a Dockerfile. Here you make CI build that image automatically, so a change that breaks the build is caught before anyone deploys it. You'll use Docker's official buildx action.

🔗 Prerequisite: This builds on the Dockerfile you created in Project 1 — Dockerize a Simple Web App.

🤖 AI Prompt

"Write a GitHub Actions job that builds my Docker image from the Dockerfile in the repo root using docker/build-push-action, without pushing it. Tag it myapp:ci."

📋 Step-by-step

Add a new job to .github/workflows/ci.yml:

  docker-build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

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

      - name: Build image (no push)
        uses: docker/build-push-action@v6
        with:
          context: .
          push: false
          tags: myapp:ci

push: false means "build only" — we verify the image compiles without uploading it anywhere yet. (Pushing comes in the next lesson.)

✅ Done when

• A docker-build job appears in the Actions tab
• The image builds successfully from your Dockerfile
• Breaking the Dockerfile turns the job red