Added CI based on GitHub Actions.

Pushes to master and stable branches will result in buildbot-like checks
on multiple OSes and architectures.

Pull requests will be checked on a public Ubuntu GitHub runner.

.github/workflows/buildbot.yml

@@ -0,0 +1,11 @@
+name: buildbot
+
+on:
+  push:
+    branches:
+      - master
+      - 'stable-1.*'
+
+jobs:
+  buildbot:
+    uses: nginx/ci-self-hosted/.github/workflows/nginx-buildbot.yml@main

.github/workflows/check-pr.yml

@@ -0,0 +1,8 @@
+name: check-pr
+
+on:
+  pull_request:
+
+jobs:
+  check-pr:
+    uses: nginx/ci-self-hosted/.github/workflows/nginx-check-pr.yml@main