BuildKit

Table of contents

Overview

BuildKit is an improved backend to replace the legacy builder. BuildKit is the default builder for users on Docker Desktop, and Docker Engine as of version 23.0.

BuildKit provides new functionality and improves your builds' performance. It also introduces support for handling more complex scenarios:

Detect and skip executing unused build stages
Parallelize building independent build stages
Incrementally transfer only the changed files in your build context between builds
Detect and skip transferring unused files in your build context
Use Dockerfile frontend implementations with many new features
Avoid side effects with rest of the API (intermediate images and containers)
Prioritize your build cache for automatic pruning

Apart from many new features, the main areas BuildKit improves on the current experience are performance, storage management, and extensibility. From the performance side, a significant update is a new fully concurrent build graph solver. It can run build steps in parallel when possible and optimize out commands that don't have an impact on the final result. We have also optimized the access to the local source files. By tracking only the updates made to these files between repeated build invocations, there is no need to wait for local files to be read or uploaded before the work can begin.

LLB

At the core of BuildKit is a Low-Level Build (LLB) definition format. LLB is an intermediate binary format that allows developers to extend BuildKit. LLB defines a content-addressable dependency graph that can be used to put together very complex build definitions. It also supports features not exposed in Dockerfiles, like direct data mounting and nested invocation.

Everything about execution and caching of your builds is defined in LLB. The caching model is entirely rewritten compared to the legacy builder. Rather than using heuristics to compare images, LLB directly tracks the checksums of build graphs and content mounted to specific operations. This makes it much faster, more precise, and portable. The build cache can even be exported to a registry, where it can be pulled on-demand by subsequent invocations on any host.

LLB can be generated directly using a golang client package that allows defining the relationships between your build operations using Go language primitives. This gives you full power to run anything you can imagine, but will probably not be how most people will define their builds. Instead, most users would use a frontend component, or LLB nested invocation, to run a prepared set of build steps.

Frontend

A frontend is a component that takes a human-readable build format and converts it to LLB so BuildKit can execute it. Frontends can be distributed as images, and the user can target a specific version of a frontend that is guaranteed to work for the features used by their definition.

For example, to build a Dockerfile with BuildKit, you would use an external Dockerfile frontend.

Getting started

BuildKit is the default builder for users on Docker Desktop and Docker Engine v23.0 and later.

If you have installed Docker Desktop, you don't need to enable BuildKit. If you are running a version of Docker Engine version earlier than 23.0, you can enable BuildKit either by setting an environment variable, or by making BuildKit the default setting in the daemon configuration.

To set the BuildKit environment variable when running the docker build command, run:

$ DOCKER_BUILDKIT=1 docker build .

Note

Buildx always uses BuildKit.

To use Docker BuildKit by default, edit the Docker daemon configuration in /etc/docker/daemon.json as follows, and restart the daemon.

{
  "features": {
    "buildkit": true
  }
}

If the /etc/docker/daemon.json file doesn't exist, create new file called daemon.json and then add the following to the file. And restart the Docker daemon.

BuildKit on Windows

Warning

BuildKit only fully supports building Linux containers. Windows container support is experimental, and is tracked in moby/buildkit#616.

BuildKit has experimental support for Windows containers (WCOW) as of version 0.13. This section walks you through the steps for trying it out. We appreciate any feedback you submit by opening an issue here, especially buildkitd.exe.

Known limitations

BuildKit on Windows currently only supports the containerd worker. Support for non-OCI workers is tracked in moby/buildkit#4836.

Prerequisites

Architecture: amd64, arm64 (binaries available but not officially tested yet).
Supported OS: Windows Server 2019, Windows Server 2022, Windows 11.
Base images: ServerCore:ltsc2019, ServerCore:ltsc2022, NanoServer:ltsc2022. See the compatibility map here.
Docker Desktop version 4.29 or later

Steps

Note

The following commands require administrator (elevated) privileges in a PowerShell terminal.

Enable the Hyper-V and Containers Windows features.
> Enable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V, Containers -All
If you see RestartNeeded as True, restart your machine and re-open a PowerShell terminal as an administrator. Otherwise, continue with the next step.
Switch to Windows containers in Docker Desktop.

Select the Docker icon in the taskbar, and then Switch to Windows containers....
Install containerd version 1.7.7 or later following the setup instructions here.

Download and extract the latest BuildKit release.

$version = "v0.13.1" # specify the release version, v0.13+
$arch = "amd64" # arm64 binary available too
curl.exe -LO https://github.com/moby/buildkit/releases/download/$version/buildkit-$version.windows-$arch.tar.gz
# there could be another `.\bin` directory from containerd instructions
# you can move those
mv bin bin2
tar.exe xvf .\buildkit-$version.windows-$arch.tar.gz
## x bin/
## x bin/buildctl.exe
## x bin/buildkitd.exe

Install BuildKit binaries on PATH.

# after the binaries are extracted in the bin directory
# move them to an appropriate path in your $Env:PATH directories or:
Copy-Item -Path ".\bin" -Destination "$Env:ProgramFiles\buildkit" -Recurse -Force
# add `buildkitd.exe` and `buildctl.exe` binaries in the $Env:PATH
$Path = [Environment]::GetEnvironmentVariable("PATH", "Machine") + `
    [IO.Path]::PathSeparator + "$Env:ProgramFiles\buildkit"
[Environment]::SetEnvironmentVariable( "Path", $Path, "Machine")
$Env:Path = [System.Environment]::GetEnvironmentVariable("Path","Machine") + ";" + `
    [System.Environment]::GetEnvironmentVariable("Path","User")

Start the BuildKit daemon.
> buildkitd.exe
In another terminal with administrator privileges, create a remote builder that uses the local BuildKit daemon.

Note

This requires Docker Desktop version 4.29 or later.
> docker buildx create --name buildkit-exp --use --driver=remote npipe:////./pipe/buildkitd buildkit-exp

Verify the builder connection by running docker buildx inspect.

> docker buildx inspect

The output should indicate that the builder platform is Windows, and that the endpoint of the builder is a named pipe.

Name:          buildkit-exp
 Driver:        remote
 Last Activity: 2024-04-15 17:51:58 +0000 UTC
 Nodes:
 Name:             buildkit-exp0
 Endpoint:         npipe:////./pipe/buildkitd
 Status:           running
 BuildKit version: v0.13.1
 Platforms:        windows/amd64
...

Create a Dockerfile and build a hello-buildkit image.

> mkdir sample_dockerfile
> cd sample_dockerfile
> Set-Content Dockerfile @"
FROM mcr.microsoft.com/windows/nanoserver:ltsc2022
USER ContainerAdministrator
COPY hello.txt C:/
RUN echo "Goodbye!" >> hello.txt
CMD ["cmd", "/C", "type C:\\hello.txt"]
"@
Set-Content hello.txt @"
Hello from BuildKit!
This message shows that your installation appears to be working correctly.
"@

Build and push the image to a registry.

> docker buildx build --push -t <username>/hello-buildkit .

After pushing to the registry, run the image with docker run.
> docker run <username>/hello-buildkit