Avoid writing to /etc/passwd unless needed

Port #461 by mac-chaffee to v4 branch.

cmd/git-sync/main.go

@@ -29,6 +29,7 @@ import (
 	"os"
 	"os/exec"
 	"os/signal"
+	"os/user"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -772,6 +773,10 @@ func handleError(log *logging.Logger, printUsage bool, format string, a ...inter
 // Put the current UID/GID into /etc/passwd so SSH can look it up.  This
 // assumes that we have the permissions to write to it.
 func addUser() error {
+	// Skip if the UID already exists. The Dockerfile already adds the default UID/GID.
+	if _, err := user.LookupId(strconv.Itoa(os.Getuid())); err == nil {
+		return nil
+	}
 	home := os.Getenv("HOME")
 	if home == "" {
 		cwd, err := os.Getwd()