trigger workflow on pull

Author: plexoos

.github/workflows/build-push-spack-buildcache.yaml

@@ -4,6 +4,13 @@ on:
   workflow_dispatch:
   schedule:
     - cron: '0 6 * * 1'
+  pull_request:
+    branches:
+      - main
+    paths:
+      - Dockerfile
+      - .github/workflows/build-push-spack-buildcache.yaml
+      - .github/workflows/build-pull-request-spack.yaml
   push:
     branches:
       - main