github actions: prints available platforms

Author: petethepig

.github/workflows/docker-dev-images.yml

@@ -34,6 +34,10 @@ jobs:
         id: buildx
         uses: docker/setup-buildx-action@v1
 
+      -
+        name: Available platforms
+        run: echo ${{ steps.buildx.outputs.platforms }}
+
       - name: Build and push
         id: docker_build
         uses: docker/build-push-action@v2