cmd/serve: add serve docker command (#5415)

Fixes #4750

Co-authored-by: Ivan Andreev <ivandeex@gmail.com>

cmd/serve/docker/unix.go

@@ -0,0 +1,56 @@
+// +build linux freebsd
+
+package docker
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"path/filepath"
+)
+
+func newUnixListener(path string, gid int) (net.Listener, string, error) {
+	// try systemd socket activation
+	fds := systemdActivationFiles()
+	switch len(fds) {
+	case 0:
+		// fall thru
+	case 1:
+		listener, err := net.FileListener(fds[0])
+		return listener, "", err
+	default:
+		return nil, "", fmt.Errorf("expected only one socket from systemd, got %d", len(fds))
+	}
+
+	// create socket outselves
+	if filepath.Ext(path) == "" {
+		path += ".sock"
+	}
+	if !filepath.IsAbs(path) {
+		path = filepath.Join(sockDir, path)
+	}
+
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return nil, "", err
+	}
+	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
+		return nil, "", err
+	}
+
+	listener, err := net.Listen("unix", path)
+	if err != nil {
+		return nil, "", err
+	}
+
+	if err = os.Chmod(path, 0660); err != nil {
+		return nil, "", err
+	}
+	if os.Geteuid() == 0 {
+		if err = os.Chown(path, 0, gid); err != nil {
+			return nil, "", err
+		}
+	}
+
+	// we don't use spec file with unix sockets
+	return listener, path, nil
+}