Run docker-image workflow when PRs on main are merged

.github/workflows/docker-image.yml

@@ -4,9 +4,15 @@ on:
     branches: main
     tags:
       - '*'
+  pull_request:
+     types:
+       closed
+     branches:
+       - 'main'
 
 jobs:
   push_to_registry:
+    if:  github.event.pull_request.merged == true
     name: Push Docker image to GitHub Packages
     runs-on: ubuntu-latest
     steps:
@@ -43,4 +49,4 @@ jobs:
           platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
-          labels: ${{ steps.meta.outputs.labels }}
+          labels: ${{ steps.meta.outputs.labels }}