fix: use Alpine container image with shellcheck support

.gitea/workflows/syntax-check.yml

@@ -20,9 +20,12 @@ on:
 jobs:
   syntax-check:
     runs-on: linux,x86_64,docker
+    container:
+      image: alpine:3.19
     steps:
       - name: Checkout code
         run: |
+          apk add --no-cache git
           git clone --depth 1 "$(echo "${{ gitea.server_url }}" | sed "s|://|://oauth2:${{ gitea.token }}@|")/${{ gitea.repository }}.git" .
           git checkout -q ${{ github.sha }}
           echo "✓ Checkout complete"
@@ -31,7 +34,7 @@ jobs:
       - name: Install shellcheck
         run: |
           echo "Installing shellcheck..."
-          apt-get update && apt-get install -y shellcheck
+          apk add --no-cache shellcheck
           echo "✓ shellcheck installed"
 
       - name: Run bash syntax check
@@ -49,7 +52,7 @@ jobs:
       - name: Verify script is executable
         run: |
           echo "Checking executable permission..."
-          if [[ -x slackecho ]]; then
+          if [ -x slackecho ]; then
             echo "✓ slackecho is executable"
           else
             echo "✗ slackecho is NOT executable"