docs: add remaining docs back with some additions/improvements

docs/kubernetes_runner.md

@@ -0,0 +1,38 @@
+# Kubernetes Runner Setup
+
+Using the GitLab Kubernetes runner allows your CI jobs to run as pods in a Kubernetes cluster.
+Nix GitLab CI can be integrated with this setup, and using advanced configuration options like
+`pod_spec` makes it easy to add runner specific caching.
+
+Using this Runner configuration ...
+
+```toml
+[[runners.kubernetes.pod_spec]]
+name = "nix-ci-cache-secrets"
+patch = '''
+  containers:
+  - name: build
+    envFrom:
+    - secretRef:
+      name: nix-ci-cache-env
+'''
+```
+
+... and a secret containing ...
+
+```yaml
+NIX_CI_RUNNER_CACHE_STRATEGY: attic
+ATTIC_SERVER: <in-cluster-url>  # example: http://atticd.<ns>.svc.cluster.local:8080
+ATTIC_CACHE: ci  # name however you want, just needs to exist
+ATTIC_TOKEN: <token>
+```
+
+... makes your jobs automatically cache their Nix store paths to the in-cluster
+attic when running with this runner.
+
+Other runners could use cachix or no cache, you get the idea ;P
+
+!!! note
+
+    This of course works with any executor where you can set environment
+    variables. This is just an example how to do it in Kubernetes easily.