Working with Docker Buildx (experimental)

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This is an experimental feature.

Experimental features provide early access to future product functionality. These features are intended for testing and feedback only as they may change between releases without warning or can be removed entirely from a future release. Experimental features must not be used in production environments. Docker does not offer support for experimental features. For more information, see Experimental features.

To enable experimental features in the Docker CLI, edit the config.json file and set experimental to enabled.

To enable experimental features from the Docker Desktop menu, click Settings (Preferences on macOS) > Daemon and then select the Experimental features check box.

Overview

Docker Buildx is a CLI plugin that extends the docker command with the full support of the features provided by Moby BuildKit builder toolkit. It provides the same user experience as docker build with many new features like creating scoped builder instances and building against multiple nodes concurrently.

Install

Docker Buildx is included in Docker 19.03 and is also bundled with the following Docker Desktop releases. Note that you must enable the ‘Experimental features’ option to use Docker Buildx.

  • Docker Desktop Enterprise version 2.1.0
  • Docker Desktop Edge version 2.0.4.0 or higher

You can also download the latest buildx binary from the Docker buildx repository.

Build with buildx

To start a new build, run the command docker buildx build .

$ docker buildx build .
[+] Building 8.4s (23/32)
 => ...

Buildx builds using the BuildKit engine and does not require DOCKER_BUILDKIT=1 environment variable to start the builds.

The docker buildx build command supports features available for docker build, including the new features in Docker 19.03 such as outputs configuration, inline build caching, and specifying target platform. In addition, Buildx also supports new features that are not yet available for regular docker build like building manifest lists, distributed caching, and exporting build results to OCI image tarballs.

You can run Buildx in different configurations that are exposed through a driver concept. Currently, Docker supports a “docker” driver that uses the BuildKit library bundled into the docker daemon binary, and a “docker-container” driver that automatically launches BuildKit inside a Docker container.

The user experience of using Buildx is very similar across drivers. However, there are some features that are not currently supported by the “docker” driver, because the BuildKit library which is bundled into docker daemon uses a different storage component. In contrast, all images built with the “docker” driver are automatically added to the “docker images” view by default, whereas when using other drivers, the method for outputting an image needs to be selected with --output.

Work with builder instances

By default, Buildx uses the “docker” driver if it is supported, providing a user experience very similar to the native docker build. Note that you must use a local shared daemon to build your applications.

Buildx allows you to create new instances of isolated builders. You can use this to get a scoped environment for your CI builds that does not change the state of the shared daemon, or for isolating builds for different projects. You can create a new instance for a set of remote nodes, forming a build farm, and quickly switch between them.

You can create new instances using the docker buildx create command. This creates a new builder instance with a single node based on your current configuration.

To use a remote node you can specify the DOCKER_HOST or the remote context name while creating the new builder. After creating a new instance, you can manage its lifecycle using the inspect, stop and rm commands. To list all available builders, use ls. After creating a new builder you can also append new nodes to it.

To switch between different builders use docker buildx use <name>. After running this command, the build commands will automatically use this builder.

Docker 19.03 also features a new docker context command that you can use to provide names for remote Docker API endpoints. Buildx integrates with docker context to ensure all the contexts automatically get a default builder instance. You can also set the context name as the target when you create a new builder instance or when you add a node to it.

Build multi-platform images

BuildKit is designed to work well for building for multiple platforms and not only for the architecture and operating system that the user invoking the build happens to run.

When you invoke a build, you can set the --platform flag to specify the target platform for the build output, (for example, linux/amd64, linux/arm64, darwin/amd64).

When the current builder instance is backed by the “docker-container” driver, you can specify multiple platforms together. In this case, it builds a manifest list which contains images for all of the specified architectures. When you use this image in docker run or docker service, Docker picks the correct image based on the node’s platform.

You can build multi-platform images using three different strategies that are supported by Buildx and Dockerfiles:

  1. Using the QEMU emulation support in the kernel
  2. Building on multiple native nodes using the same builder instance
  3. Using a stage in Dockerfile to cross-compile to different architectures

QEMU is the easiest way to get started if your node already supports it (for example. if you are using Docker Desktop). It requires no changes to your Dockerfile and BuildKit automatically detects the secondary architectures that are available. When BuildKit needs to run a binary for a different architecture, it automatically loads it through a binary registered in the binfmt_misc handler.

Using multiple native nodes provide better support for more complicated cases that are not handled by QEMU and generally have better performance. You can add additional nodes to the builder instance using the --append flag.

# assuming contexts node-amd64 and node-arm64 exist in "docker context ls"
$ docker buildx create --use --name mybuild node-amd64
mybuild
$ docker buildx create --append --name mybuild node-arm64
$ docker buildx build --platform linux/amd64,linux/arm64 .

Finally, depending on your project, the language that you use may have good support for cross-compilation. In that case, multi-stage builds in Dockerfiles can be effectively used to build binaries for the platform specified with --platform using the native architecture of the build node. A list of build arguments like BUILDPLATFORM and TARGETPLATFORM is available automatically inside your Dockerfile and can be leveraged by the processes running as part of your build.

FROM --platform=$BUILDPLATFORM golang:alpine AS build
ARG TARGETPLATFORM
ARG BUILDPLATFORM
RUN echo "I am running on $BUILDPLATFORM, building for $TARGETPLATFORM" > /log
FROM alpine
COPY --from=build /log /log

High-level build options

Buildx also aims to provide support for high-level build concepts that go beyond invoking a single build command.

BuildKit efficiently handles multiple concurrent build requests and deduplicating work. The build commands can be combined with general-purpose command runners (for example, make). However, these tools generally invoke builds in sequence and therefore cannot leverage the full potential of BuildKit parallelization, or combine BuildKit’s output for the user. For this use case, we have added a command called docker buildx bake.

The bake command supports building images from compose files, similar to a compose build, but allowing all the services to be built concurrently as part of a single request.

Set buildx as the default builder

Running the command docker buildx install sets up docker builder command as an alias to docker buildx. This results in the ability to have docker build use the current buildx builder.

To remove this alias, run docker buildx uninstall.

Docker, buildx, multi-arch