ci: allow manual trigger for Docker Hub README updates (#150)

- Add workflow_dispatch to enable manual triggering from GitHub Actions UI
- Fix branch name from master to main

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: LaurenceJJones

.github/workflows/update_docker_hub_doc.yml

@@ -1,9 +1,10 @@
-name: (push-master) Update Docker Hub README
+name: Update Docker Hub README
 
 on:
+  workflow_dispatch:
   push:
     branches:
-      - master
+      - main
     paths:
       - 'docker/README.md'