CI build times haven't improved despite the team adding multi-stage builds to every Dockerfile

Builds are still slow even though final images are smaller. Diagnose other likely bottlenecks. Expected reasoning: multi-stage builds primarily affect final image size/content, not build cache efficiency directly — check whether instruction ordering within the builder stage is still cache-unfriendly (e.g., copying full source before installing dependencies), whether the CI runners are ephemeral with no persisted Docker layer cache between runs (requiring registry-based or platform-native cache import/export, Chapter 10), and whether BuildKit cache mounts are being used for package manager down