ci: re-enable ci-cpp container (arm64 image now in registry)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/cpp-tests.yml

@@ -11,17 +11,12 @@ on:
 jobs:
   test:
     runs-on: eulernest
-    # container image intentionally omitted until ci-cpp:latest is
-    # rebuilt for linux/arm64 and pushed to git.eulernest.eu
+    container:
+      image: git.eulernest.eu/conformallab/ci-cpp:latest
 
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install build tools
-        run: |
-          sudo apt-get update -qq
-          sudo apt-get install -y --no-install-recommends cmake g++ git ca-certificates
-
       - name: Configure (tests-only mode)
         run: cmake -S code -B build -DCMAKE_BUILD_TYPE=Release