How would you speed up slow Docker builds in a CI pipeline?

Use multi-stage builds with deliberate instruction ordering (Chapter 3) so unchanged layers (dependencies) are cached; wire up CI-platform-native layer caching (GitHub Actions' type=gha cache, or a registry-based cache with --cache-from/--cache-to) so cache persists across separate job runs, since each CI job otherwise starts from a clean environment with no prior build cache available.