debug actions build's docker context

Author: baronfel

.github/workflows/dotnet.yml

@@ -26,5 +26,7 @@ jobs:
         global-json-file: 'global.json' # in addition to the 6.0 we need for tests, install "latest"
     - name: Build
       run: dotnet build --consoleloggerparameters:NoSummary --warnAsError
+    - name: Discover docker platforms
+      run: docker buildx ls
     - name: Test
       run: dotnet test --no-build --logger GitHubActions