FROM scratch and multarch buildx build using BUILDPLATFORM arg bug

[matejvasek]

Hello everyone, I have a trouble using multistage multiarch build when using scratch base.

It does not work with moby 26.1.3 in GH Action.
However it works on my fedora with moby 24.0.5 and also with podman

I think it might be a bug in moby.

I have Dockerfile like:

FROM --platform=$BUILDPLATFORM scratch AS builder

ARG BUILDPLATFORM
ARG BUILDARCH
ARG TARGETARCH

ADD .artifacts/alpine-minirootfs-$BUILDARCH.tar.gz /

COPY .artifacts/go.tar.gz /tmp/go.tar.gz
RUN tar -C /usr/local -xzf /tmp/go.tar.gz ### THIS  FAILS
ENV PATH="/usr/local/go/bin:$PATH"

WORKDIR /workspace

COPY go.mod go.sum ./
RUN go mod download -x

COPY . .

RUN GOARCH=$TARGETARCH go build -o func-util -trimpath -ldflags '-w -s' ./cmd/func-util

#########################

FROM scratch

ARG TARGETARCH

ADD .artifacts/alpine-minirootfs-$TARGETARCH.tar.gz /

COPY --from=builder /workspace/func-util /usr/local/bin/

RUN apk add --no-cache tar ### THIS WORKS

The issues is that execution of any binary in the builder stage causes an error:

#12 0.179 .buildkit_qemu_emulator: /bin/sh: Invalid ELF image for this architecture

That's really strange since builder should be native and no emulation should be involved at all. IMO there is a bug with platform and scratch base. If replace scratch -> alpine then build work. But the binaries should be the same no matter what is in alpine since I am overriding it at ADD .artifacts/alpine-minirootfs-$BUILDARCH.tar.gz /. This make absolutely no sense to me.

Most importantly: why it works with older moby and podman?

Also note that the emulation works as expected for the final stage. The apk binary runs just fine.

[matejvasek]

I suspect that moby might look into declared architecture of base image and scratch has none. But that's just my wild guess.

[matejvasek]

It looks like moby tries to run qemu emulation on native binaries for some reason.

[thaJeztah]

Let me transfer this to the BuildKit repository, which is where the builder is maintained

Possibly related (?)

• Fix scratch multi-platform images #4526
• feat: allow setting platform for scratch image #5670

[dsseng]

Yes, it's related to my PR #5670, I faced and proposed a solution to the same problem

[matejvasek]

Yes, it looks related.

[matejvasek]

I fixed this issue in my GH Action by using setup-qemu-action in my flow, however I think it should work even without it.