chore(ci): run backend tests through dotnet SDK container

.gitea/workflows/deploy-socialize.yml

@@ -7,12 +7,18 @@ on:
 
 jobs:
   test:
-    runs-on: dotnet
+    runs-on: ubuntu-latest
     steps:
-      - name: Install checkout dependencies
-        run: apt-get update && apt-get install -y git nodejs
       - uses: actions/checkout@v4
-      - run: dotnet test backend/Socialize.slnx
+      - name: Install Docker CLI
+        run: apt-get update && apt-get install -y docker.io
+      - name: Test backend
+        run: |
+          docker run --rm \
+            -v "$PWD:/workspace" \
+            -w /workspace \
+            mcr.microsoft.com/dotnet/sdk:10.0 \
+            dotnet test backend/Socialize.slnx
 
   image:
     needs: test