Bumping dockerfile build go version to 1.21 (#677)

Author: i3149

Dockerfile

@@ -1,5 +1,5 @@
 # build ktranslate
-FROM golang:1.20-alpine as build
+FROM golang:1.21-alpine as build
 RUN apk add -U libpcap-dev alpine-sdk bash libcap
 COPY . /src
 WORKDIR /src