fix: use shell-based checkout for self-hosted runner

actions/checkout@v4 needs Node.js which isn't available in host mode.
Use git clone directly instead.

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

.gitea/workflows/build.yml

@@ -9,12 +9,14 @@ jobs:
     runs-on: self-hosted
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        run: |
+          rm -rf workspace && mkdir workspace
+          git clone --depth 1 --branch main http://gitea:3000/mwf975_git/tippspiel.git workspace
 
       - name: Build Docker Image
         run: |
-          docker build -t wm2026-tippspiel:latest -t wm2026-tippspiel:${{ gitea.sha }} .
+          cd workspace
+          docker build -t wm2026-tippspiel:latest -t wm2026-tippspiel:${GITHUB_SHA:-latest} .
 
-      - name: Restart Container
-        run: |
-          docker compose -f /tmp/tippspiel-compose.yml up -d --force-recreate 2>/dev/null || echo "Stack not yet deployed via Portainer"
+      - name: Cleanup
+        run: rm -rf workspace