ci: Setting up conditional CI runs depending on PR comment trigger

.github/workflows/pr-keyword-check.yml

@@ -0,0 +1,10 @@
+name: PR keyword check
+on:
+  workflow_dispatch:
+
+# This workflow deliberately does nothing of value. The branch rules require the associated "Check PR issue comments for trigger keywords" job to have run before the PR being merged
+jobs:
+  pr-keyword-check:
+    runs-on: unity-linux-runner
+    steps:
+      - run: ls

.yamato/_triggers.yml

@@ -15,6 +15,9 @@
 # Focuses on critical validation paths that we should validate before merging PRs
 # Cancels previous runs on new commits
 # Excludes draft PRs
+# Excludes running when changes ONLY touching documentation files
+# Requires `/ci ngo` or `/ci ignore` comment to trigger the job. This was implemented and explained in https://github.com/Unity-Technologies/com.unity.netcode.gameobjects/pull/3577
+
 
 # Nightly:
 # This test validates same subset as pull_request_trigger with addition of mobile/console tests and webgl builds
@@ -38,6 +41,20 @@
 
 #-----------------------------------------------------------------------------------
 
+# Gates the merger of PRs as a pre-requisite. Runs when `/ci ngo` or `/ci ignore` is present as an issue comment
+# Notice that I needed this job to do "something" and that's why there is a placeholder GitHub action used
+# TODO: In the future we could have comment like "docs" or "project" to run a specific subset of tests
+check_pr_issue_comments_for_trigger_keywords:
+  name: Check PR for trigger comments of ngo or ignore. For example /ci ngo
+  agent:
+    type: Unity::github::action
+    action_source: pr-keyword-check.yml
+  triggers:
+    expression: |-
+      pull_request.comment eq "ngo" OR
+      pull_request.comment eq "ignore"
+
+
 # Run all relevant tasks when a pull request targeting the develop or release branch is created or updated.
 # In order to have better coverage we run desktop standalone tests with different configurations which allows to mostly cover for different platforms, scripting backends and editor versions.
 # This job will FIRST run "run_quick_checks" jobs (defined in _run-all.yml) since it's the dependency of project pack jobs which is on the lowest dependency tier. This runs the fastest checks (like PVP or code standards) and ONLY IF those pass it will run the rest of the tests.
@@ -60,15 +77,10 @@ pull_request_trigger:
     - .yamato/desktop-standalone-tests.yml#desktop_standalone_test_testproject_win_il2cpp_6000.0
     - .yamato/cmb-service-standalone-tests.yml#cmb_service_standalone_test_testproject_ubuntu_il2cpp_trunk
   triggers:
+    expression: |-
+      pull_request.comment eq "ngo" AND
+      NOT pull_request.changes.all match "**/Documentation~/**"
     cancel_old_ci: true
-    pull_requests:
-      - targets:
-          only:
-            - "develop"
-            - "develop-2.0.0"
-            - "/release\/.*/"
-      - drafts: false
-
 
 # Run all tests on trunk on nightly basis.
 # Same subset as pull_request_trigger with addition of mobile/desktop/console tests and webgl builds