ci: use Octave 8.4.0 container for release gate tests

The release gate tests were using apt-get octave (older version) which
caused test_to_step_function to fail. Use the same gnuoctave/octave:8.4.0
container as the main test workflow for consistency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: HanSur94

.github/workflows/release.yml

@@ -12,12 +12,10 @@ jobs:
   test:
     name: Gate Tests
     runs-on: ubuntu-latest
+    container: gnuoctave/octave:8.4.0
     steps:
       - uses: actions/checkout@v6
 
-      - name: Install Octave and Xvfb
-        run: sudo apt-get update && sudo apt-get install -y octave octave-dev xvfb
-
       - name: Run tests
         run: |
           xvfb-run octave --eval "cd('tests'); r = run_all_tests(); if r.failed > 0; exit(1); end"