init docker build workflow

.github/workflows/docker-build.yaml

@@ -0,0 +1,17 @@
+name: Build and push Docker image
+on:
+  workflow_dispatch:
+  pull_request:
+    branches:
+      - main
+    types:
+      - closed
+
+jobs:
+  if_merged:
+    if: github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    steps:
+    - run: |
+        echo The PR was merged
+