fix: update logging of Docker registry credentials to use secrets

.gitea/workflows/build.yml

@@ -29,8 +29,8 @@ jobs:
 
       - name: log the docker registry login credentials (for debugging)
         run: |
-          echo "REGISTRY_USERNAME: $REGISTRY_USERNAME"
-          echo "REGISTRY_PASSWORD: ${REGISTRY_PASSWORD:+[REDACTED]}"
+          echo "REGISTRY_USERNAME: ${{ secrets.REGISTRY_USERNAME }}"
+          echo "REGISTRY_PASSWORD: ${{ secrets.REGISTRY_PASSWORD }}"
 
       - name: Log in to Gitea container registry
         run: |