fix: stop the job's docker socket from becoming a directory (#1215)

Fixes https://gitea.com/gitea/runner/issues/1213

Fix the DooD regression that mounts `/var/run/docker.sock` as a directory. Keep the Docker proxy available through job and post steps. Clean stale resources before opening it, then remove containers before their networks and volumes during teardown.

Use a unique filesystem probe and preserve socket ownership. Fall back to direct access when proxying is unsupported. Preserve exec output and clean up active streams and failed starts.

Add a real Docker job test for mounted socket access, post steps and resource cleanup.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1215
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

act/container/docker_proxy_unix.go

@@ -0,0 +1,27 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !WITHOUT_DOCKER && (linux || darwin || netbsd)
+
+package container
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+func copyDockerSocketPermissions(socket string, info os.FileInfo) error {
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return errors.New("docker socket ownership is unavailable")
+	}
+	if err := os.Chown(socket, int(stat.Uid), int(stat.Gid)); err != nil {
+		return err
+	}
+	if err := os.Chown(filepath.Dir(socket), int(stat.Uid), -1); err != nil {
+		return err
+	}
+	return os.Chmod(socket, info.Mode().Perm())
+}