CI: add rust/ build+test workflow

.gitea/workflows/build.yml

@@ -0,0 +1,19 @@
+name: build
+on: [push, workflow_dispatch]
+
+jobs:
+  build-test:
+    runs-on: docker
+    container: rust:1-bookworm
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        run: git clone --depth 1 ${{ github.server_url }}/${{ github.repository }}.git .
+
+      - name: Build (rust/)
+        run: cargo build --quiet
+        working-directory: rust
+
+      - name: Test (rust/)
+        run: cargo test --quiet
+        working-directory: rust