fix: update workflow to trigger on main branch

Author: iamjr15

.github/workflows/docker-build.yml

@@ -3,7 +3,7 @@ name: Build and Deploy to Cloud Run
 on:
   push:
     branches:
-      - master
+      - main
     paths:
       - 'backend/**'
       - '.github/workflows/docker-build.yml'