php: Use correct Dockerfile in Alpine build

.github/workflows/php.yml

@@ -80,7 +80,7 @@ jobs:
       - uses: docker/build-push-action@v5
         id: build
         with:
-          context: ./${{ env.image }}/
+          context: ./${{ env.image }}/alpine/
           push: ${{ github.event_name != 'pull_request' }}
           tags: ghcr.io/${{ env.organization }}/${{ env.image }}:alpine,ghcr.io/${{ env.organization }}/${{ env.image }}:${{ steps.extract_version.outputs.version }}-alpine
           platforms: linux/amd64,linux/arm64