CI/CD: building multiarch Docker images

GitLab CI/CD
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Hi,

I already asked this question few days ago on StackOverflow, but since I got no answer, I’m posting it here.

So, here it is:

I want an easy way to build multiarch Docker images in a GitLab runner. By easy, I mean that I just would have to add a .gitlab-ci.yml in my project and it would work.

Here is the .gitlab-ci.yml that I wrote. It builds a multiarch image using buildx and then pushes it to the GitLab registry:

image: cl00e9ment/buildx

services:
- name: docker:dind

variables:
  PLATFORMS: linux/amd64,linux/arm64
  TAG: latest

before_script:
  - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" "$CI_REGISTRY"

build:
  stage: build
  script:
  - docker buildx build --platform "$PLATFORMS" -t "${CI_REGISTRY_IMAGE}:${TAG}" . --push

The problem is that the linux/arm64 platform isn’t available.

Here is how I built the cl00e9ment/buildx image (strongly inspired from snadn/docker-buildx):

Here is the Dockerfile:

FROM docker:latest

ENV DOCKER_CLI_EXPERIMENTAL=enabled
ENV DOCKER_HOST=tcp://docker:2375/

RUN mkdir -p ~/.docker/cli-plugins \
    && wget -qO- https://api.github.com/repos/docker/buildx/releases/latest | grep "browser_download_url.*linux-amd64" | cut -d : -f 2,3 | tr -d '"' | xargs wget -O ~/.docker/cli-plugins/docker-buildx \
    && chmod a+x ~/.docker/cli-plugins/docker-buildx
RUN docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
RUN docker context create buildx \
    && docker buildx create buildx --name mybuilder \
    && docker buildx use mybuilder
RUN docker buildx inspect --bootstrap

…add here is the .gitlab-ci.yml file used to build and push the cl00e9ment/buildx image:

image: docker:latest

services:
  - name: docker:dind

before_script:
  - docker login -u cl00e9ment -p "$DOCKER_HUB_TOKEN"

build:
  stage: build
  script:
  - docker build --add-host docker:`grep docker /etc/hosts | awk 'NR==1{print $1}'` --network host -t cl00e9ment/buildx .
  - docker run --add-host docker:`grep docker /etc/hosts | awk 'NR==1{print $1}'` --network host cl00e9ment/buildx docker buildx inspect --bootstrap
  - docker push cl00e9ment/buildx

test:
  stage: test
  script:
  - docker run --add-host docker:`grep docker /etc/hosts | awk 'NR==1{print $1}'` --network host cl00e9ment/buildx docker buildx inspect --bootstrap

So what’s happening?

  • At the end of the build, in the Dockerfile, I run docker buildx inspect --bootstrap to list the available platforms. It gives linux/amd64, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/arm/v7, linux/arm/v6. So it’s all good.
  • After that, I run it again (just after the build and just before the push) and it still gives linux/amd64, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/arm/v7, linux/arm/v6.
  • However, on the test stage, when the image is freshly downloaded from Docker Hub on a clean environment, it gives linux/amd64, linux/386.

Why?
Thank you.