Make platforms an input

.github/workflows/_build.yml

@@ -22,6 +22,11 @@ on:
         required: false
         type: string
         default: 'ubuntu-latest'
+      platforms:
+        description: 'Platforms to build for, comma separated list of arch/os pairs'
+        required: false
+        type: string
+        default: 'linux/amd64,linux/arm64'
     secrets:
       username:
         description: 'username at registry'
@@ -34,20 +39,33 @@ env:
   REGISTRY_IMAGE: ${{ inputs.image }}
 
 jobs:
+  convert:
+    runs-on: ${{ inputs.runs-on }}
+    outputs:
+      platforms: ${{ steps.convert.outputs.platforms }}
+    steps:
+      -
+        name: Convert Platforms
+        id: convert
+        run: |
+          platforms=${{ inputs.platforms }}
+          inside=$(printf %s\\n "$platforms" | sed -E -e 's/,/\", \"/g' -e 's/\" +/\"/g' -e 's/ +\"/\"/g')
+          printf 'platforms=["%s"]' "$inside" >> $GITHUB_OUTPUT
+
   build:
     runs-on: ${{ inputs.runs-on }}
+    needs:
+      - convert
     strategy:
       fail-fast: false
       matrix:
-        platform:
-          - linux/amd64
-          - linux/arm64
+        platform: ${{ needs.convert.outputs.platforms }}
     steps:
       -
         name: Prepare
         id: platform
         run: |
-          platform=${{ matrix.platform }}
+          platform=${{ needs.convert.outputs.platforms }}
           echo "PLATFORM_PAIR=${platform//\//-}" >> $GITHUB_ENV
       -
         name: Checkout
@@ -77,7 +95,7 @@ jobs:
         uses: docker/build-push-action@v5
         with:
           context: .
-          platforms: ${{ matrix.platform }}
+          platforms: ${{ needs.convert.outputs.platforms }}
           labels: ${{ steps.meta.outputs.labels }}
           outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true
           file: ${{ inputs.file }}