Update docker-publish.yml

only trigger when there are backend service code changes.

.github/workflows/docker-publish.yml

@@ -5,6 +5,10 @@ name: Docker build and publish
 on:
   push:
     branches: ['main']
+    paths:
+      - Dockerfile
+      - main.py
+      - requirements.txt
 
 # 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: