ci: pin docker/build-push-action v6.15.0 SHA

Author: nschonni

.github/workflows/build-test.yml

@@ -69,7 +69,7 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Build image
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4 # v6.15.0
         with:
           push: false
           load: true