Update "tianon/gosu" Docker Hub image to build via bashbrew instead of bespoke script

This gives us nice provenance, etc; see https://explore.ggcr.dev/?image=tianon/gosu:1.16

Author: tianon

hub/alpine.yml

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+preferredOrder=( alpine debian )
+
+dir="$(dirname "$BASH_SOURCE")"
+cd "$dir"
+
+commit="$(git log -1 --format='format:%H' HEAD -- .)"
+cat <<-EOH
+	Maintainers: Tianon Gravi <tianon@tianon.xyz> (@tianon)
+	GitRepo: https://github.com/tianon/gosu.git
+	GitCommit: $commit
+	Directory: hub
+	Builder: buildkit
+EOH
+
+version=
+i=0; jq=; froms=()
+for variant in "${preferredOrder[@]}"; do
+	from="$(awk 'toupper($1) == "FROM" { print $2; exit }' "Dockerfile.$variant")" # TODO multi-stage?
+	variantVersion="$(awk 'toupper($1) == "ENV" && toupper($2) == "GOSU_VERSION" { print $3; exit }' "Dockerfile.$variant")"
+	version="${version:-$variantVersion}"
+	if [ "$version" != "$variantVersion" ]; then
+		echo >&2 "error: mismatched version in '$variant' ('$version' vs '$variantVersion')"
+		exit 1
+	fi
+	jq="${jq:+$jq, }$variant: (.[$i].arches | keys_unsorted)"
+	froms["$i"]="$from"
+	(( i++ )) || :
+done
+arches="$(bashbrew remote arches --json "${froms[@]}" | jq -sc "{ $jq }")" # { alpine: [ "amd64", ... ], debian: [ "amd64", ... ] }
+
+queue="$(jq <<<"$arches" -r 'to_entries | map(@sh "variant=\(.key)\narch=\(.value[])") | map(@sh) | join("\n")')"
+eval "queue=( $queue )"
+
+declare -A seenArches=()
+for item in "${queue[@]}"; do
+	eval "$item" # variant=yyy arch=xxx
+	[ -n "$variant" ]
+	[ -n "$arch" ]
+	tags="$variant-$arch"
+	sharedTags="$variant, $version-$variant"
+	if [ -z "${seenArches["$arch"]:-}" ]; then
+		tags+=", $arch"
+		sharedTags+=", $version, latest"
+	fi
+	echo
+	echo "Tags: $tags"
+	[ -z "$sharedTags" ] || echo "SharedTags: $sharedTags"
+	echo "Architectures: $arch"
+	echo "File: Dockerfile.$variant"
+	: "${seenArches["$arch"]:=1}"
+done