add a new github workflow to run when PRs are merged

- this workflow should result in userenv images that are built and
  pushed to the production quay.io repo

.github/workflows/crucible-merged.yaml

@@ -0,0 +1,26 @@
+name: crucible-merged
+
+on:
+  pull_request_target:
+    types: [ closed ]
+    branches: [ master ]
+    paths-ignore:
+    - LICENSE
+    - '**.md'
+    - .github/workflows/faux-unittest.yaml
+    - .github/workflows/faux-crucible-ci.yaml
+    - 'docs/**'
+    - engine/engine-script-library.py
+    - engine/engine-script.py
+  workflow_dispatch:
+
+jobs:
+  call-core-crucible-ci_if-merged
+    if: github.event.pull_request.merged == true:
+    uses: perftool-incubator/crucible-ci/.github/workflows/core-crucible-ci.yaml@main
+    with:
+      ci_target: "rickshaw"
+      ci_target_branch: "master"
+      github_workspace: "$GITHUB_WORKSPACE"
+    secrets:
+      production_registry_auth: ${{ secrets.CRUCIBLE_PRODUCTION_ENGINES_REGISTRY_AUTH }}