Docker layer caching in GitHub Actions with cache-from and cache-to

Every CI run rebuilds Docker images from scratch, wasting 3-10 minutes pulling base images and reinstalling dependencies. Without layer caching, even a one-line change triggers a full rebuild.

Use BuildKit's inline cache or registry cache with docker/build-push-action:

- uses: docker/setup-buildx-action@v3

- uses: docker/build-push-action@v5
  with:
    context: .
    push: false
    tags: myapp:latest
    cache-from: type=gha
    cache-to: type=gha,mode=max

For registry-based caching (better for self-hosted runners):

- uses: docker/build-push-action@v5
  with:
    cache-from: type=registry,ref=ghcr.io/org/myapp:cache
    cache-to: type=registry,ref=ghcr.io/org/myapp:cache,mode=max

BuildKit stores individual layer blobs. mode=max caches intermediate layers too, not just the final image. The gha backend uses GitHub's cache API; registry backend survives runner restarts and works across forks.