fix: downgrade checkout to v4 and add Docker mirror to build.yaml

- Use actions/checkout@v4 to avoid punycode deprecation warning
- Add docker-mirror.mcintire.me configuration for faster builds

.forgejo/workflows/build.yaml

@@ -18,10 +18,14 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@v4
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
+        with:
+          buildkitd-config-inline: |
+            [registry."docker.io"]
+              mirrors = ["docker-mirror.mcintire.me"]
 
       - name: Log in to container registry
         uses: docker/login-action@v3