balena build --arch armv7hf produces amd64 binaries when FROM is a multi-arch image (Docker Engine 29)

[schubydoo]

Expected Behavior

balena build --deviceType raspberrypi3 --arch armv7hf should produce an image containing ARM binaries when the FROM is a multi-arch official Docker image (e.g. alpine:3.23).

Actual Behavior

The build completes successfully, but the resulting image contains x86_64 binaries. The amd64 manifest of the base image is selected regardless of --arch.

Steps to Reproduce

Dockerfile:

ARG ALPINE_VERSION=3.23
FROM alpine:${ALPINE_VERSION}
RUN echo "uname -m: $(uname -m)" && \
    apk add --no-cache file >/dev/null && \
    file /bin/busybox

Build command:

balena build --deviceType raspberrypi3 --arch armv7hf --noparent-check

Build log (relevant lines):

[Info]    Building for armv7hf/raspberrypi3
[Info]    Docker Desktop detected (daemon architecture: "x86_64")
[Build]   main Step 2/X : FROM alpine:${ALPINE_VERSION}
[Build]   main  ---> 5b10f432ef3d
[Build]   main uname -m: x86_64
[Build]   main /bin/busybox: ELF 64-bit LSB pie executable, x86-64, ...

Image ID 5b10f432ef3d is the amd64 variant of alpine:3.23.

Verification that the host environment is correct

Direct Docker test on the same machine, same daemon, returns the correct architecture:

$ docker run --rm --platform=linux/arm/v7 alpine:3.23 uname -m
armv7l

This rules out binfmt, QEMU (trying with --emulated parameter), and Docker Desktop's emulation setup as the cause.

Specifications

• balena CLI version: 24.1.1 (also tried v24.1.4, v25.0.0, and v25.1.3)

• Docker Client Version:
  Version: 29.4.2
  API version: 1.54
  Go version: go1.26.2
  Git commit: 055a478
  Built: Fri May 1 01:27:10 2026
  OS/Arch: windows/amd64
  Context: desktop-linux

• Docker Server Version:
  Server: Docker Desktop 4.72.0 (225998)
  Engine:
  Version: 29.4.2
  API version: 1.54 (minimum version 1.40)
  Go version: go1.26.2
  Git commit: d329809
  Built: Fri May 1 01:24:36 2026
  OS/Arch: linux/amd64
  Experimental: false
  containerd:
  Version: v2.2.3
  GitCommit: 77c84241c7cbdd9b4eca2591793e3d4f4317c590
  runc:
  Version: 1.3.5
  GitCommit: v1.3.5-0-g488fc13e
  docker-init:
  Version: 0.19.0
  GitCommit: de40ad0

• OS: Windows 11 + WSL2

• Install method: standalone installer

• Reproducible after: full Docker Desktop factory reset

Related

• PR Add multiarch support #2301 mentions adding support after multiple issues raised (i.e. balena build / deploy --build selects base image of wrong architecture (multiarch image manifest) #1508)

Workarounds confirmed

• Pinning the FROM line to an arm/v7 digest (FROM alpine@sha256:...) produces a correct ARM build.
• balena push <fleet> on cloud builders is unaffected.