Use newer golang/alpine version

bump golang to 1.21.6
bump alpine to 3.19

Dockerfile

@@ -1,5 +1,5 @@
 # syntax = docker/dockerfile:latest
-FROM golang:1.20.4-alpine3.17 as build
+FROM golang:1.21.6-alpine3.19 as build
 RUN apk add --update build-base gcc wget git libpcap-dev
 WORKDIR /app
 COPY src/* /app/