feat: add GitLab CI example

docs/k8s/examples/gitlab-ci.md

@@ -0,0 +1,69 @@
+---
+title: "GitLab CI"
+description: "Demo running GitLab CI with Imageless Kubernetes"
+---
+
+You can use Imageless Kubernetes in many applications where you would rely on a conventional image-based workflow.
+
+This example shows how you can use Imageless Kubernetes with GitLab CI, running jobs inside of the same Flox environment you use for development.
+
+## GitLab runner configuration
+
+To configure your GitLab runner to run Imageless Kubernetes pods, add this section to the runner's `config.toml`:
+
+```toml
+[[runners]]
+  [runners.kubernetes]
+    namespace = {% raw %}"{{ default .Release.Namespace .Values.runners.jobNamespace }}"{% endraw %}
+    runtime_class_name = "flox"
+    image = "flox/empty:1.0.0"
+    pod_annotations_overwrite_allowed = '^flox\.dev.*'
+    [runners.kubernetes.pod_annotations]
+      "flox.dev/skip-containers" = "init-permissions,helper"
+      "flox.dev/skip-containers-exec" = "build"
+      "flox.dev/activate-mode" = "dev" # optional
+```
+
+!!! note "Note"
+    These options can also be passed as part of the job definition in `.gitlab-ci.yml`, see the [GitLab documentation][gitlab-k8s-docs] for more details.
+
+These settings will start all GitLab job pods using the Flox runtime, with an empty container image, and allow setting additional annotations (e.g. `flox.dev/environment`) on a per-job basis.
+
+The `flox.dev/skip-containers` and `flox.dev/skip-containers-exec` annotations are necessary to allow GitLab's init containers to get the code and job script into build container for execution.
+
+`flox.dev/activate-mode` is set to make build dependencies available to the job script.
+
+See the [configuration][config] page for more details on annotations.
+
+## GitLab job configuration
+
+Once you've configured the runner, you will need to supply each job with the desired `flox.dev/environment` annotation, which can be done in `.gitlab-ci.yml`:
+
+```yaml
+stages:
+  - hello
+
+hello-job:
+  stage: hello
+  variables:
+    KUBERNETES_POD_ANNOTATIONS_1: "flox.dev/environment=flox/hello"
+  script:
+    - hello
+```
+
+where the value of the annotation is the name of an environment from [FloxHub][floxhub].
+
+!!! note "Note"
+    The `flox.dev/environment` annotation is *not* optional -- pods will fail to start if it is not supplied.
+
+## Conclusion
+
+Now, any job you target to this runner will be executed with Imageless Kubernetes.
+
+If you utilize the same Flox environment used for development, you will be able to seamlessly test with the exact same packages, regardless of what system or architecture is used.
+
+Since the CI environment doesn’t rely on a container image, updates are instant: run `flox push`, and the job will pick up the changes automatically.
+
+[gitlab-k8s-docs]: https://docs.gitlab.com/runner/executors/kubernetes/
+[config]: ../config.md
+[floxhub]: ../../concepts/floxhub.md