Enable manual workflow dispatch for Docker publish

Added the `workflow_dispatch` event trigger to the Docker publish demo workflow file. This allows the workflow to be started manually from the GitHub Actions interface in addition to being triggered by a push event.

.github/workflows/docker-publish-demo.yml

@@ -6,6 +6,7 @@ name: Publish Demo Containers to Ghcr.io
 # documentation.
 
 on:
+  workflow_dispatch:
   push:
     # Publish semver tags as releases.
     tags: [ 'v*.*.*', 'demo' ]