Fix: Make Dockerhub publish dependent on npm publish (#123)

Currently, there is a race where the Dockerhub publish completes before
the npm one does, causing the image to use the older version of the test
package.

Reference: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run

.github/workflows/docker_publish.yml

@@ -1,7 +1,10 @@
 name: docker image publish
 on:
-  release:
-    types: [published]
+  workflow_run:
+    workflows: ["npm publish"]
+    branches: [main]
+    types:
+      - completed
 
 jobs:
   build_and_push_docker_image: