fix(docker): docker build error (#1274)

Dockerfile

@@ -10,10 +10,7 @@ ENV REPOSITORY github.com/future-architect/vuls
 COPY . $GOPATH/src/$REPOSITORY
 RUN cd $GOPATH/src/$REPOSITORY && make install
 
-
-FROM alpine:3.13
-
-LABEL maintainer hikachan sadayuki-matsuno
+FROM alpine:3.14
 
 ENV LOGDIR /var/log/vuls
 ENV WORKDIR /vuls

logging/logutil.go

@@ -100,7 +100,7 @@ func NewCustomLogger(debug, quiet, logToFile bool, logDir, logMsgAnsiColor, serv
 			}
 		}
 	} else if quiet {
-		log.Out = io.Discard
+		log.Out = ioutil.Discard
 	} else {
 		log.Out = os.Stderr
 	}