docs: add Docker build workflow to manual build section (6.2)

gitea-python-ci-cd.md

@@ -568,6 +568,25 @@ jobs:
 
 > **Note:** `actions/upload-artifact` stores artifacts temporarily (default retention: 90 days). For permanent distribution, use the release workflow with a `v*` tag.
 
+**Docker-only projects** don't need artifact uploads — just verify the image builds:
+
+```yaml
+# .gitea/workflows/build.yml
+name: Build Docker Image
+
+on:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build Docker image
+        run: docker build -t your-app:dev .
+```
+
 ## 7. Docker Build
 
 ### Multi-stage Dockerfile