Build Docker image on release rather than on branch (#561)

* Build image on release

* Allow workflow to be trigger manually outside of releases

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

@@ -1,10 +1,11 @@
 #
 name: Create and publish a Docker image
 
-# Configures this workflow to run every time a change is pushed to the branch called `release`.
+# Configures this workflow to run every time a release is published.
 on:
-  push:
-    branches: ['release']
+  workflow_dispatch:
+  release:
+    types: [published]
 
 # Defines two custom environment variables for the workflow. These are used for the Container registry domain, and a name for the Docker image that this workflow builds.
 env: