ci: run Docker tests only when the Dockerfiles are modified

.github/workflows/docker.yml

@@ -7,6 +7,8 @@ on:
       - '[0-9]+.[0-9]+.[0-9]+'
   pull_request:
     branches: [ main ]
+    paths:
+      - 'packaging/docker/**'
 jobs:
   docker-images:
     name: Docker Images