Merge branch 'renovate/docker.io-library-golang-1.x' into 'main'

chore(deps): update docker.io/library/golang docker tag to v1.21.0

See merge request !39

Dockerfile

-FROM docker.io/library/golang:1.20.7 as build-env
+FROM docker.io/library/golang:1.21.0 as build-env
 
 # renovate: datasource=git-tags depName=https://git.shivering-isles.com/github-mirror/AdguardTeam/dnsproxy.git
 ARG DNSPROXY_VERSION=v0.54.0