Docker Multiarch

The purpose of this article is to build and run images for different architectures.

The examples were run on an arm64 M1 macbook with docker desktop installed.

Installing emulators

Attempting to run an amd64 container on an arm64 machine without an emulator results in:

$ docker run --platform linux/amd64 --rm amd64/alpine uname -mo
standard_init_linux.go:228: exec user process caused: exec format error

We will install and run our emulators using tonistiigi/binfmt.

docker run --privileged --rm tonistiigi/binfmt --install all   # everything
docker run --privileged --rm tonistiigi/binfmt --install amd64 # specific

And now, if I run the amd64 container, it works:

$ docker run --platform linux/amd64 --rm amd64/alpine uname -mo
x86_64 Linux

Building

We will start with a trivial DockerFile:

FROM alpine:3.17.1
CMD uname -mo

When we use buildx we need to also specify an output type. In this case, we are targetting the development machine’s docker registry.

docker buildx build --platform linux/amd64 -t uname:amd64 . --output type=docker

And we can run this new container:

$ docker run --platform=linux/amd64 uname:amd64
x86_64 Linux

It is also important to note that this only works because the alpine:3.17.1 image supports our target architecture. If we build for an unsupported platform, we get an error:

$ docker buildx build --platform linux/riscv64 -t uname:riscv64 . --output type=docker
...
error: failed to solve: alpine:3.17.1: no match ...