Container Image Layer Caching in GitHub Actions

Deep dive into Docker BuildKit's cache mount feature, registry-backed cache layers with --cache-to/--cache-from, and multi-stage build patterns specifically optimized for CI/CD environments where ephemeral runners destroy local caches. Covers practical patterns for monorepo setups with shared base layers, dependency-layer splitting for Node/JVM/Python stacks, and the GitHub Actions cache backend integration that avoids the 10GB cache eviction cliff.