Update build-image.yaml, build docker image on release publish (#289)

Does this look right for also building a docker image when we publish a
release? (now that the version number isn't changed as a commit)

.github/workflows/build-image.yaml

@@ -3,6 +3,8 @@ on:
   push:
     branches:
       - main
+  release:
+    types: [published]
 
 # This job uses RafikFarhad's GitHub action to build and
 # push a docker image to a specified GCP repository