add matrix passing debug step

.github/workflows/build.yml

@@ -162,6 +162,15 @@ jobs:
           username: ${{ inputs.REGISTRY_USER != '' && inputs.REGISTRY_USER || secrets.REGISTRY_USER != '' && secrets.REGISTRY_USER || vars.REGISTRY_USER != '' && vars.REGISTRY_USER || github.actor }}
           password: ${{ secrets.REGISTRY_TOKEN }}
 
+      - name: Show matrix payload received by this job
+        run: |
+          echo "Expression toJson(matrix): ${{ toJson(matrix) }}"
+          echo "Expression matrix.dir: '${{ matrix.dir }}'"
+          echo "ENV MATRIX_DIR: '${MATRIX_DIR}'"
+          echo "Dockerfile path expression: '${{ matrix.dir }}/Dockerfile'"
+        env:
+          MATRIX_DIR: ${{ matrix.dir }}
+
       - name: Derive image name and tags
         id: meta
         shell: bash