Add .net9

Author: xyzjesper

.github/workflows/docker.yml

@@ -28,7 +28,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: docker/setup-qemu-action@v3
-
+      - uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: '9.0.x'
       - uses: docker/setup-buildx-action@v3
         with:
           buildkitd-flags: --debug