07 — Speed up builds with caching

Category: Optimization · Priority: Medium · Step: 8

Cache npm dependencies and Docker layers so repeat runs take seconds, not minutes.

🎯 What is this task?

CI re-downloads and rebuilds everything on a clean machine each time — which is slow. Caching stores expensive results between runs. You'll cache two things: npm packages (already easy via setup-node) and Docker build layers (via GitHub Actions cache).

🤖 AI Prompt

"Add GitHub Actions caching to my Docker build so unchanged layers are reused between runs using cache-from and cache-to of type gha. Explain how layer caching saves time."

📋 Step-by-step

npm cache — you already have it from lesson 02:

      - uses: actions/setup-node@v4
        with:
          node-version: "20"
          cache: "npm"   # caches ~/.npm automatically

Docker layer cache — add to your build-push-action step:

      - uses: docker/build-push-action@v6
        with:
          context: .
          push: true
          tags: ghcr.io/${{ github.repository }}:latest
          cache-from: type=gha
          cache-to: type=gha,mode=max

• cache-from: type=gha → pull cached layers from GitHub's cache.
• cache-to: type=gha,mode=max → save all layers (not just the final image) for next time.

The second run reuses unchanged layers and finishes far faster.

✅ Done when

• cache: "npm" is set on setup-node
• The Docker build uses cache-from/cache-to: type=gha
• A second run is noticeably faster than the first