armhf: fix FTBFS on armhf

Closes: #455

Patch by Shizuo Fujita <[email protected]>

The armhf build was failing with an execve error (exit code 127) during apk add curl.
Seems it is compatibility issues between recent Alpine Linux versions (time64 syscalls) and QEMU emulation.

Similar with https://gitlab.alpinelinux.org/alpine/aports/-/issues/17775

The failure was occurred in builder stage.
For file downloads, we can use the same architecture as the host.

Signed-off-by: Kentaro Hayashi <[email protected]>

Author: kenhys

Dockerfile.template.erb

@@ -23,7 +23,7 @@ FROM alpine:3.19
 <% else %>
 <% if is_armhf %>
 # To set multiarch build for Docker hub automated build.
-FROM golang:alpine AS builder
+FROM --platform=$BUILDPLATFORM golang:alpine AS builder
 WORKDIR /go
 ENV QEMU_DOWNLOAD_SHA256 47ae430b0e7c25e1bde290ac447a720e2ea6c6e78cd84e44847edda289e020a8
 RUN apk add curl --no-cache