merge branch 'pr-1548'

LGTMs: @crosbymichael @mrunalp @cyphar
Closes #1548

libcontainer/user/user.go

@@ -358,8 +358,8 @@ func GetExecUser(userSpec string, defaults *ExecUser, passwd, group io.Reader) (
 
 				// Okay, so it's numeric. We can just roll with this.
 			}
-		} else if len(groups) > 0 && uidErr != nil {
-			// Supplementary group ids only make sense if in the implicit form for non-numeric users.
+		} else if len(groups) > 0 {
+			// Supplementary group ids only make sense if in the implicit form.
 			user.Sgids = make([]int, len(groups))
 			for i, group := range groups {
 				user.Sgids[i] = group.Gid