ci: Add manual trigger for container build workflow

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

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

.gitea/workflows/build-container.yml

@@ -4,9 +4,12 @@
 name: Build Container
 
 on:
+  # Trigger on version tags
   push:
     tags:
       - 'v[0-9]+.[0-9]+.[0-9]+'
+  # Allow manual trigger from Gitea UI
+  workflow_dispatch:
 
 jobs:
   build: