Test against Bash 3.2, 4.4, 5.0, 5.3 in CI (#177)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/bats.yaml

@@ -4,10 +4,16 @@ on: [push, pull_request]
 jobs:
   test:
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        bash-version: ['3.2', '4.4', '5.0', '5.3']
+    container:
+      image: bash:${{ matrix.bash-version }}
     steps:
-      - name: Install Bats
-        uses: bats-core/bats-action@3.0.1
       - name: Check out repository
         uses: actions/checkout@v5
+      - name: Install Bats
+        run: apk add --no-cache bats
       - name: Run tests
         run: bats test