Document GitLab Runner Executor (#463)

docs/integrations/gitlab-runner.md

@@ -0,0 +1,49 @@
+# GitLab Runner Executor
+
+It is possible to run GitLab jobs in isolated ephemeral Tart Virtual Machines via [Tart Executor](https://github.com/cirruslabs/gitlab-tart-executor).
+Tart Executor utilizes [custom executor](https://docs.gitlab.com/runner/executors/custom.html) feature of GitLab Runner.
+
+# Basic Configuration
+
+Configuring Tart Executor for GitLab Runner is as simple as installing `gitlab-tart-executor` binary from Homebrew:
+
+```bash
+brew install cirruslabs/cli/gitlab-tart-executor
+```
+
+And updating configuration of your self-hosted GitLab Runner to use `gitlab-tart-executor` binary:
+
+```toml
+concurrent = 2
+
+[[runners]]
+  # ...
+  executor = "custom"
+  builds_dir = "/Users/admin/builds" # directory inside the 
+  cache_dir = "/Users/admin/cache"
+  [runners.feature_flags]
+    FF_RESOLVE_FULL_TLS_CHAIN = false
+  [runners.custom]
+    prepare_exec = "gitlab-tart-executor"
+    prepare_args = ["prepare"]
+    run_exec = "gitlab-tart-executor"
+    run_args = ["run"]
+    cleanup_exec = "gitlab-tart-executor"
+    cleanup_args = ["cleanup"]
+```
+
+Now you can use Tart Images in your `.gitlab-ci.yml`:
+
+```yaml
+# You can use any remote Tart Image.
+# Tart Executor will pull it from the registry and use it for creating ephemeral VMs.
+image: ghcr.io/cirruslabs/macos-ventura-base:latest
+
+test:
+  tags:
+    - tart-installed # in case you tagged runners with Tart Executor installed
+  script:
+    - uname -a
+```
+
+For more advanced configuration please refer to [GitLab Tart Executor repository](https://github.com/cirruslabs/gitlab-tart-executor).

mkdocs.yml

@@ -87,6 +87,7 @@ nav:
   - "Quick Start": quick-start.md
   - "Integrations":
     - "GitHub Actions": integrations/github-actions.md
+    - "GitLab Runner": integrations/gitlab-runner.md
     - "Self-hosted CI": integrations/cirrus-cli.md
     - "Managing VMs": integrations/vm-management.md
   - "Licensing": licensing.md