fix: create temp directory for skopeo in CI

skopeo needs /var/tmp or /tmp for temporary files when copying
docker-archive images. Use --tmpdir flag to explicitly use /tmp.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.forgejo/workflows/build-image.yml

@@ -23,15 +23,20 @@ jobs:
       - name: Push to registry
         if: github.ref == 'refs/heads/main'
         run: |
+          # Ensure temp directory exists for skopeo
+          mkdir -p /var/tmp /tmp
+
           image=$(nix build --no-link --print-out-paths .#solidhaus-image)
           skopeo copy \
             --dest-tls-verify=false \
+            --tmpdir /tmp \
             "docker-archive:$image" \
             "docker://registry.toph.so/solidhaus:latest"
 
           # Also tag with commit SHA
           skopeo copy \
             --dest-tls-verify=false \
+            --tmpdir /tmp \
             "docker-archive:$image" \
             "docker://registry.toph.so/solidhaus:${GITHUB_SHA:0:7}"