5.2.1.3 BuildKit Cache Support
A focused guide to BuildKit Cache Support, connecting core concepts with practical Docker and container operations.
BuildKit cache support refers to the more advanced caching capabilities BuildKit introduced beyond the legacy builder's simple layer cache, including cache mounts for persisting package manager downloads across builds and exportable cache that can be shared between different build machines entirely.
Cache Mounts for Package Manager Downloads
A cache mount lets a directory's contents persist across builds independently of the layer cache itself, useful for package manager download caches that should survive even when the layer that uses them must be rebuilt.
RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt
Even if this RUN instruction's layer must be rebuilt due to a changed requirements.txt, the underlying pip download cache persists, meaning previously downloaded packages don't need to be re-fetched from the network.
Exporting and Importing Cache Across Machines
BuildKit supports exporting the build cache to external storage (a registry, a local directory) and importing it on a different machine, enabling cache reuse across separate CI runners or build agents that don't share local disk state.
docker buildx build --cache-to type=registry,ref=myregistry/myapp:cache --cache-from type=registry,ref=myregistry/myapp:cache -t myapp .
This allows a CI pipeline running on ephemeral, disposable build agents to still benefit from cache built up by previous, entirely separate build runs.
Why This Matters Beyond a Single Local Machine
Without shared, exportable cache, every CI build on a fresh agent would need to rebuild everything from scratch, losing all the caching benefit a developer's local machine naturally accumulates over repeated builds.
docker buildx build --cache-from type=registry,ref=myregistry/myapp:cache -t myapp .
Why BuildKit's Advanced Cache Support Matters
These capabilities extend caching benefits beyond what a single machine's local layer cache alone could provide, making fast, cache-aware builds practical even in distributed, ephemeral CI environments where the legacy builder's local-only caching model would otherwise provide little benefit.