Add name output to identify builder instance name

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

setup-buildx/README.md

@@ -77,6 +77,9 @@ jobs:
         uses: docker/actions/setup-buildx@v1
         with:
           buildx-version: latest
+      -
+        name: Builder instance name
+        run: echo ${{ steps.buildx.outputs.name }}
       -
         name: Available platforms
         run: echo ${{ steps.buildx.outputs.platforms }}
@@ -102,6 +105,7 @@ Following outputs are available
 
 | Name          | Type    | Description                           |
 |---------------|---------|---------------------------------------|
+| `name`        | String  | Builder instance name |
 | `platforms`   | String  | Available platforms (comma separated) |
 
 ### environment variables