feat: add Kubernetes backend (#1260)

Adds a Kubernetes backend. A `name:kubernetes://image` label runs the job as one pod through the Kubernetes API, with services as containers of the same pod. Step env is exported by a script sent over exec stdin, keeping it out of the pod spec and process arguments, and service env goes into a per-job Secret. Docker actions and container options are rejected. See `docs/kubernetes.md`.

Operators shape job pods with `kubernetes.pod_template`, with per-label `pod_templates` merged in `runs-on` order and a `$services` entry for service containers. The idle cleanup removes pods and Secrets a runner left behind.

The client calls the Kubernetes API directly and runs step commands over `coder/websocket` (same dependency used in gitea) instead of pulling in the heavyweight `kubernetes/client-go`. Includes an e2e test against k3s.

Replaces: https://gitea.com/gitea/runner/pulls/1000
Fixes: https://gitea.com/gitea/runner/issues/31

Assisted-by: Claude Code:claude-opus-5-5
Co-authored-by: Zettat123 <[email protected]>
Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1260
Reviewed-by: Lunny Xiao <[email protected]>
Reviewed-by: Zettat123 <[email protected]>
Co-authored-by: silverwind <[email protected]>

internal/app/run/runner_test.go

@@ -245,6 +245,16 @@ func TestRunnerFallbackPlatform(t *testing.T) {
 	}
 }
 
+func TestRunnerKubernetesOptionsLayerTheLabelPodTemplatesInRunsOnOrder(t *testing.T) {
+	cfg := &config.Config{Kubernetes: config.Kubernetes{
+		PodTemplate:  map[string]any{"base": true},
+		PodTemplates: map[string]map[string]any{"gpu": {"gpu": true}, "large": {"large": true}, "other": {"other": true}},
+	}}
+	r := &Runner{cfg: cfg, labels: labels.Labels{{Name: "linux"}, {Name: "gpu"}, {Name: "large"}}}
+
+	require.Equal(t, []map[string]any{{"base": true}, {"large": true}, {"gpu": true}}, r.kubernetesOptions([]string{"linux", "large", "other", "gpu", "large"}).PodTemplates)
+}
+
 // Proxy variables are assembled per task, because a job's service containers have to be
 // reached directly and they are only known once the workflow is parsed.
 func TestNewRunnerLeavesProxyToTheTask(t *testing.T) {
@@ -350,7 +360,7 @@ func TestNewRunnerCacheServiceV2(t *testing.T) {
 	r.isolatedCacheContainer = func() string { return "a1b2c3d4e5f6" }
 	pickPlatform := func(runsOn []string) string { return map[string]string{"native": labels.SelfHostedPlatform}[runsOn[0]] }
 	for job, isolated := range map[string]bool{"native": false, "linux": true, "containerized": true, "empty": false} {
-		cacheURL, cacheContainer := r.cacheForJob(workflow.GetJob(job), pickPlatform)
+		cacheURL, cacheContainer := r.cacheForJob(workflow.GetJob(job), pickPlatform(workflow.GetJob(job).RunsOn()))
 		if isolated {
 			assert.Equal(t, "a1b2c3d4e5f6", cacheContainer, job)
 			assert.Equal(t, strings.Replace(r.cacheHandler.ExternalURL(), "127.0.0.1", cacheContainer, 1), cacheURL, job)