ci: disable Docker cache to test cmocka 2.0 breakage

Add --no-cache to docker build to force fresh image build with
latest cmocka 2.0.1 from debian:testing.

.github/workflows/ci-code.yml

@@ -31,7 +31,7 @@ jobs:
       - uses: actions/checkout@v4
       - name: Run tests
         run: |
-          docker build -f Dockerfile.${{ matrix.flavor }} -t profanity .
+          docker build --no-cache -f Dockerfile.${{ matrix.flavor }} -t profanity .
           docker run profanity ./ci-build.sh
 
   code-style: