Dockerfile: update base image from alpine:3.10 to latest
Dockerfile: update base image from alpine:3.10 to latest

we can just use latest stable alpine release (currently 3.14)

also switch build image from golang default (debian-based)
to alpine-based, it's smaller and faster to download

mreiferson/nsq,ploxiln/nsq,nsqio/nsq,bitly/nsq,ploxiln/nsq,rpau/nsq,mreiferson/nsq,mreiferson/nsq,ploxiln/nsq,bitly/nsq,rpau/nsq,nsqio/nsq,bitly/nsq,ploxiln/nsq,rpau/nsq,ploxiln/nsq,rpau/nsq,nsqio/nsq,bitly/nsq,nsqio/nsq,bitly/nsq,nsqio/nsq,mreiferson/nsq,rpau/nsq,mreiferson/nsq