Docker multi-platform build fails for linux/arm/v6

[fishgrimsby]

Description

Building the multi-platform docker image now fails for linux/arm/v6

Error messages

 > [linux/arm/v6 internal] load metadata for docker.io/library/golang:1.21:
------
Dockerfile:1
--------------------
   1 | >>> FROM golang:1.21 AS build-stage
   2 |     
   3 |     WORKDIR /app
--------------------
ERROR: failed to solve: golang:1.21: no match for platform in manifest: not found
Error: buildx failed with: ERROR: failed to solve: golang:1.21: no match for platform in manifest: not found

Steps to reproduce

Trigger the Docker workflow at: .github/workflows/docker.yaml

Alternatively, on a local machine, from the repo root run:

docker buildx build --file ./build/package/Dockerfile --platform linux/amd64,linux/arm64,linux/arm/v6,linux/arm/v7 -t borgmatic-exporter:test .

[fishgrimsby]

Seems this is an intentional breaking change - details can be found here: docker-library/golang#502 and docker-library/golang#500 (comment)

The resolution appears to be to switch to the alpine image for the build-stage