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]>

Makefile

@@ -12,6 +12,7 @@ PODMAN_TEST_IMAGE ?= quay.io/containers/podman:v5.8.4-immutable@sha256:90405e3c6
 E2E_JOB_IMAGE ?= node:24-bookworm@sha256:64af3819f9275802414d7cdc38c27e9d82bd564dec4d4da87d008255d36c63b4 # renovate: datasource=docker
 SERVICE_IMAGE ?= nginx:1.31.4-alpine@sha256:db35bfc6b2951e7f8a72db5db120288c127ffaeeb4a6d4b95a26fead017d5913 # renovate: datasource=docker
 E2E_GITEA_IMAGE ?= gitea/gitea:main-nightly
+E2E_K3S_IMAGE ?= rancher/k3s:v1.36.4-k3s1@sha256:edad48e12bf81c3a09ac1c05c0c0ffaaa22145980b989d6fae84543a76b83657 # renovate: datasource=docker
 
 LINUX_ARCHS ?= linux/amd64,linux/arm64
 DARWIN_ARCHS ?= darwin-12/amd64,darwin-12/arm64
@@ -170,7 +171,7 @@ E2E_CONCURRENCY ?= 8
 
 .PHONY: test-e2e
 test-e2e: ## run Gitea compatibility tests against E2E_GITEA_IMAGE
-	@E2E_CONCURRENCY=$(E2E_CONCURRENCY) E2E_GITEA_IMAGE=$(E2E_GITEA_IMAGE) E2E_JOB_IMAGE=$(E2E_JOB_IMAGE) GO=$(GO) SERVICE_IMAGE=$(SERVICE_IMAGE) ./tools/test-e2e.sh
+	@E2E_CONCURRENCY=$(E2E_CONCURRENCY) E2E_GITEA_IMAGE=$(E2E_GITEA_IMAGE) E2E_K3S_IMAGE=$(E2E_K3S_IMAGE) E2E_JOB_IMAGE=$(E2E_JOB_IMAGE) GO=$(GO) SERVICE_IMAGE=$(SERVICE_IMAGE) ./tools/test-e2e.sh
 
 .PHONY: install
 install: $(GOFILES) ## install the runner binary via `go install`