Build images with base image platforms

Upstream may reduce available platforms when versions reach EOL

Author: Earlopain

.github/workflows/nightly.yml

@@ -36,6 +36,8 @@ jobs:
       - run: echo '${{ needs.set-up-matrix.outputs.json }}' > versions.json
       - run: echo date=$(jq -r .master.date versions.json) >> $GITHUB_ENV
       - run: ./apply-templates.sh
+      - run: echo base=$(ruby -e 's = File.read("master/${{ matrix.variant }}/Dockerfile"); puts s[/FROM (.*)/, 1]') >> $GITHUB_ENV
+      - run: echo platforms=$(./architectures.rb $base) >> $GITHUB_ENV
 
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
@@ -58,5 +60,5 @@ jobs:
         with:
           context: ./master/${{ matrix.variant }}
           push: true
-          platforms: linux/amd64,linux/arm64,linux/arm/v7,linux/arm/v6,linux/386,linux/ppc64le,linux/s390x
+          platforms: ${{ env.platforms }}
           tags: ${{ env.tag1 }},${{ env.tag2 }}

architectures.rb

@@ -0,0 +1,16 @@
+#!/usr/bin/env ruby
+
+require "json"
+
+image = ARGV[0]
+json = JSON.parse(`docker manifest inspect #{image}`)
+
+identifiers = json["manifests"].filter_map do |manifest|
+  platform = manifest["platform"]
+  next unless platform["os"] == "linux"
+
+  optional_variant = "/#{platform['variant']}" if platform['variant']
+  "#{platform['os']}/#{platform['architecture']}#{optional_variant}"
+end
+
+puts identifiers.join(",")