Add docs about CI setup (#6397)

docs/_sidebar.md

@@ -41,4 +41,5 @@
   * [Test cases](docs/test_cases.md)
   * [Vagrant](docs/vagrant.md)
   * [CI Matrix](docs/ci.md)
+  * [CI Setup](docs/ci-setup.md)
 * [Roadmap](docs/roadmap.md)

docs/ci-setup.md

@@ -0,0 +1,20 @@
+# CI Setup
+
+## Pipeline
+
+1. unit-tests: fast jobs for fast feedback (linting, etc...)
+2. deploy-part1: small number of jobs to test if the PR works with default settings
+3. deploy-part2: slow jobs testing different platforms, OS, settings, CNI, etc...
+4. deploy-part3: very slow jobs (upgrades, etc...)
+
+## Runners
+
+Kubespray has 3 types of GitLab runners:
+
+- packet runners: used for E2E jobs (usually long)
+- light runners: used for short lived jobs
+- auto scaling runners: used for on-demand resources, see [GitLab docs](https://docs.gitlab.com/runner/configuration/autoscale.html) for more info
+
+## Vagrant
+
+Vagrant jobs are using the [quay.io/kubespray/vagrant](/test-infra/vagrant-docker/Dockerfile) docker image with `/var/run/libvirt/libvirt-sock` exposed from the host, allowing the container to boot VMs on the host.