create a workflow to build ngen image on PR that affects ngen or ngen…

…-deps

.github/workflows/docker_build_ngen.yml

@@ -0,0 +1,21 @@
+name: Build and push ngen image
+on:
+  pull_request:
+    branches: [ main, notreal ]
+    paths: 
+      - 'docker/Dockerfile.ngen'
+      - 'docker/Dockerfile.ngen-deps'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: self-hosted
+    # For pull requests, it's not necessary to checkout code
+    permissions:
+      pull-requests: read
+    steps:
+      - name: Build ngen Docker image
+        uses: ./.github/action_templates/build-and-push
+        with:
+          image-name: "ngen"
+          dockerfile-name: "Dockerfile.ngen"