Use manual git clone instead of actions/checkout

The docker:latest container doesn't have Node.js, which is
required by actions/checkout@v4. Use manual git clone instead.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.forgejo/workflows/docker-build.yml

@@ -12,7 +12,9 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        run: |
+          git clone ${{ github.server_url }}/${{ github.repository }} .
+          git checkout ${{ github.sha }}
 
       - name: Log in to Forgejo Container Registry
         run: echo "${{ secrets.GITEA_TOKEN }}" | docker login forgejo.sivic.me -u ${{ github.actor }} --password-stdin