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]>

act/container/tar.go

@@ -0,0 +1,69 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// Copyright 2020 The nektos/act Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+import (
+	"archive/tar"
+	"context"
+	"io"
+	"path/filepath"
+	"strings"
+
+	"gitea.com/gitea/runner/act/common"
+	"gitea.com/gitea/runner/act/filecollector"
+
+	"github.com/go-git/go-billy/v5/helper/polyfill"
+	"github.com/go-git/go-billy/v5/osfs"
+	"github.com/go-git/go-git/v5/plumbing/format/gitignore"
+)
+
+func writeFilesTar(ctx context.Context, w io.Writer, uid, gid int, files ...*FileEntry) error {
+	tw := tar.NewWriter(w)
+	for _, file := range files {
+		common.Logger(ctx).Debugf("Writing entry to tarball %s len:%d", file.Name, len(file.Body))
+		if err := tw.WriteHeader(&tar.Header{
+			Name: file.Name,
+			Mode: file.Mode,
+			Size: int64(len(file.Body)),
+			Uid:  uid,
+			Gid:  gid,
+		}); err != nil {
+			return err
+		}
+		if _, err := tw.Write([]byte(file.Body)); err != nil {
+			return err
+		}
+	}
+	return tw.Close()
+}
+
+// writeDirTar prefixes entries with dstPath for extraction at root.
+func writeDirTar(ctx context.Context, w io.Writer, dstPath, srcPath string, useGitIgnore, skipGitDir bool, uid, gid int) error {
+	logger := common.Logger(ctx)
+	tw := tar.NewWriter(w)
+	srcPrefix := strings.TrimSuffix(filepath.Dir(srcPath), string(filepath.Separator)) + string(filepath.Separator)
+	logger.Debugf("Stripping prefix:%s src:%s", srcPrefix, srcPath)
+
+	var ignorer gitignore.Matcher
+	if useGitIgnore {
+		ps, err := gitignore.ReadPatterns(polyfill.New(osfs.New(srcPath)), nil)
+		if err != nil {
+			logger.Debugf("Error loading .gitignore: %v", err)
+		}
+
+		ignorer = gitignore.NewMatcher(ps)
+	}
+
+	if err := filepath.Walk(srcPath, (&filecollector.FileCollector{
+		Ignorer:    ignorer,
+		SrcPath:    srcPath,
+		SrcPrefix:  srcPrefix,
+		SkipGitDir: skipGitDir,
+		Handler:    &filecollector.TarCollector{TarWriter: tw, UID: uid, GID: gid, DstDir: dstPath[1:]},
+	}).CollectFiles(ctx, []string{})); err != nil {
+		return err
+	}
+	return tw.Close()
+}