26.02.2026

BuildKit: Docker's Hidden Build Engine You Should Know About

head-image

Most DevOps engineers interact with BuildKit daily without realizing its full potential. When you run docker build, BuildKit is the engine under the hood. But it can do far more than build container images from Dockerfiles.

What is BuildKit?

BuildKit is a modern build toolkit designed around three key concepts:

LLB (Low-Level Build): A content-addressable intermediate representation that describes filesystem operations as a DAG. Think of it as LLVM IR for build systems.
Frontends: Pluggable parsers that convert your build definition (Dockerfile, YAML, custom DSL) into LLB.
Solver: Executes the LLB graph with aggressive caching and parallel execution.

The key insight is that the build language is not baked into BuildKit. The # syntax= directive at the top of Dockerfiles tells BuildKit which frontend image to use. You can write custom frontends for any format.

Key Features

Content-addressable caching: Operations are cached at the vertex level across the entire DAG, not just linear layers
Parallel execution: Independent branches of the build graph run simultaneously
Multiple output types: Export as OCI image, local directory, tarball, or custom format
Remote caching: Share build cache across CI runners via registries
Pluggable frontends: Build from Dockerfile, YAML, TOML, or any custom spec

Practical Usage

The --output flag unlocks non-image use cases:

# Export final filesystem to local directory
docker buildx build --output type=local,dest=./out .

# Export as tarball
docker buildx build --output type=tar,dest=./out.tar .

# Export as OCI image tarball
docker buildx build --output type=oci,dest=./image.tar .

This makes BuildKit useful for producing compiled binaries, packages, or documentation without creating container images.

Custom Frontends in Action

Projects like Dagger, Earthly, and Depot are built on BuildKit's LLB. You can write your own frontend that reads any spec format and emits LLB operations.

Example: building an APK package with a custom frontend:

docker buildx build \
  -f spec.yml \
  --build-arg BUILDKIT_SYNTAX=tuananh/apkbuild \
  --output type=local,dest=./out \
  .

No Dockerfile required. The frontend parses your YAML and produces the package directly.

Conclusion

BuildKit gives you a parallelized, cached, content-addressable build engine for free. If you're building CI/CD pipelines, artifact generation, or multi-step compilation workflows, consider leveraging BuildKit directly rather than reinventing caching and parallelism.

The Dockerfile is just the default frontend. The real power lies in the engine underneath.

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