Do I need this action to build for 32-bit architecture in docker on GitHub Actions

[ericoporto]

Hey,

I am using ubuntu-latest runner and came up with a pipeline that builds a docker image when it is modified, like below (I am working with legacy software and need this old compatibility)

name: Build Linux Docker Image

on:
  push:
    paths:
      - 'ci/linux/Dockerfile'
  workflow_dispatch:

permissions:
  contents: read
  packages: write

env:
  IMAGE_NAME: ghcr.io/${{ github.repository_owner }}/ags-linux-builder-image
  BRANCH_TAG: ${{ github.ref_name }}

jobs:
  build-and-push-amd64:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6

      - name: Log in to GHCR
        uses: docker/login-action@v4
        with:
          registry: ghcr.io
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}

      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v4

      - name: Build and push amd64 image
        uses: docker/build-push-action@v7
        with:
          context: ci/linux
          file: ci/linux/Dockerfile
          platforms: linux/amd64
          push: true
          tags: |
            ${{ env.IMAGE_NAME }}:${{ env.BRANCH_TAG }}-amd64
          build-args: |
            FROM_PLATFORM=linux/amd64
            FROM_DEBIAN=debian/eol:jessie
          cache-from: type=registry,ref=${{ env.IMAGE_NAME }}:${{ env.BRANCH_TAG }}-amd64-buildcache
          cache-to:   type=registry,ref=${{ env.IMAGE_NAME }}:${{ env.BRANCH_TAG }}-amd64-buildcache,mode=max

  build-and-push-i386:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6

      - name: Log in to GHCR
        uses: docker/login-action@v4
        with:
          registry: ghcr.io
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}

      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v4

      - name: Build and push i386 image
        uses: docker/build-push-action@v7
        with:
          context: ci/linux
          file: ci/linux/Dockerfile
          platforms: linux/386
          push: true
          tags: |
            ${{ env.IMAGE_NAME }}:${{ env.BRANCH_TAG }}-i386
          build-args: |
            FROM_PLATFORM=linux/386
            FROM_DEBIAN=debian/eol:jessie
          cache-from: type=registry,ref=${{ env.IMAGE_NAME }}:${{ env.BRANCH_TAG }}-i386-buildcache
          cache-to:   type=registry,ref=${{ env.IMAGE_NAME }}:${{ env.BRANCH_TAG }}-i386-buildcache,mode=max

Would I need to ad something like

      - name: Set up QEMU
        uses: docker/setup-qemu-action@v4
        with:
          platforms: linux/386

On the 32-bit pipeline? Or I am misunderstanding the linux/386 platform?

[gistrec]

For linux/386 on a GitHub-hosted ubuntu-latest runner, you normally do not need docker/setup-qemu-action

linux/386 is still x86. On an amd64 Linux runner, 32-bit x86 userspace can usually run through the host kernel's native compatibility support, so BuildKit does not need QEMU in the same way it would for a different ISA such as linux/arm64, linux/arm/v7, linux/ppc64le, linux/riscv64, or linux/s390x

QEMU is mainly needed when the build has to execute binaries for a non-native architecture. That is the common case for amd64 runners building ARM, PowerPC, RISC-V, s390x, etc

So for your linux/386 job, this should usually be enough:

- uses: docker/setup-buildx-action@v3

- uses: docker/build-push-action@v6
  with:
    platforms: linux/386

A couple of caveats:

• The base image and packages must actually support linux/386. If the base image has no i386 variant, the build will fail regardless of QEMU.
• If your Dockerfile uses custom platform build args such as FROM_PLATFORM, make sure they are wired correctly. BuildKit's standard automatic args are BUILDPLATFORM, TARGETPLATFORM, TARGETOS, TARGETARCH, and TARGETVARIANT.

For linux/386 only, I would skip setup-qemu-action. Add it only if you also build non-x86 platforms