Modify Docker workflow to include backend path filters

Updated workflow to trigger on changes in the 'backend' directory for both push and pull request events.

Author: Warren-SJ

.github/workflows/docker-image.yml

@@ -4,6 +4,13 @@ on:
   push:
     branches:
       - main
+    paths:
+      - 'backend/**'
+  pull_request:
+    branches:
+      - main
+    paths:
+      - 'backend/**'
   workflow_dispatch:
 
 jobs: