Run arm64 builds natively.

In principle we could just cross-build from a single runner, but it's
less hassle to just do the same thing we do for the Ruby builds where we
don't cross-compile.

Dockerfile

@@ -3,7 +3,7 @@ ARG go_version=1.22
 ARG go_tag_suffix=-alpine
 
 
-FROM ${go_registry}golang:${go_version}${go_tag_suffix} AS builder
+FROM --platform=$TARGETPLATFORM $${go_registry}golang:${go_version}${go_tag_suffix} AS builder
 ARG TARGETARCH TARGETOS
 ARG GOARCH=$TARGETARCH GOOS=$TARGETOS
 ARG CGO_ENABLED=0
@@ -16,7 +16,7 @@ COPY . ./
 RUN go build -o /bin/govuk-exporter main.go
 
 
-FROM scratch
+FROM --platform=$TARGETPLATFORM $scratch
 COPY --from=builder /bin/govuk-exporter /bin/govuk-exporter
 COPY --from=builder /usr/share/ca-certificates /usr/share/ca-certificates
 COPY --from=builder /etc/ssl /etc/ssl