Use non-global logger to avoid unexpected global log discarding

Reported in GoogleContainerTools/kaniko#1955

Changes in
awslabs#309 made it
possible for callers depending on this code as a Go library to redirect
log output to ioutil.Discard to avoid spurious log statements.

Unfortunately, because in that change the logger being used was the
shared global logger.StandardLogger(), redirecting its output to
ioutil.Discard to quiet ecr-login also disabled all logging using the
shared global logger (like Kaniko does).

ecr-login/ecr.go

@@ -48,7 +48,7 @@ func WithLogOutput(w io.Writer) Option {
 func NewECRHelper(opts ...Option) *ECRHelper {
 	e := &ECRHelper{
 		clientFactory: api.DefaultClientFactory{},
-		logger:        logrus.StandardLogger(),
+		logger:        logrus.New(),
 	}
 	for _, o := range opts {
 		o(e)