Avoid returning the UID when resolving the GIDs. (#2689)

* Fix GetUIDAndGIDFromString test.

* Avoid returning by default the UID when resolving the GIDs.

pkg/util/syscall_credentials.go

@@ -19,6 +19,7 @@ package util
 import (
 	"fmt"
 	"strconv"
+	"strings"
 	"syscall"
 
 	"github.com/pkg/errors"
@@ -54,6 +55,12 @@ func SyscallCredentials(userStr string) (*syscall.Credential, error) {
 		groups = append(groups, uint32(i))
 	}
 
+	if !(len(strings.Split(userStr, ":")) > 1) {
+		if u.Gid != "" {
+			gid, _ = getGID(u.Gid)
+		}
+	}
+
 	return &syscall.Credential{
 		Uid:    uid,
 		Gid:    gid,