Dockerfile: Clarify port mapping
Dockerfile: Clarify port mapping

* Extend comment about port mapping since everyone seems to want to run
  on port 443/tcp.
* Accept that nobody (except the already competent) will read the
  comment and ask anyway.

caervs/docker-openvpn,rdev02/docker-openvpn,kylemanna/docker-openvpn,richnologies/docker-openvpn,hanjaelee/docker-openvpn,elie195/docker-openvpn,mouyigang/docker-openvpn,myplacedk/docker-openvpn,gl-sergei/docker-openvpn,morcavon/docker-openvpn,slamont/docker-openvpn,hjnijlunsing/docker-openvpn-armhf,wernerb/docker-openvpn,neingeist/docker-openvpn,alessandro-aglietti/docker-openvpn,csakoda/docker-openvpn,jvassev/docker-openvpn,StefanGuelland/docker-openvpn,pushrax/docker-openvpn,jeffcao/docker-openvpn,dx9/docker-openvpn,shanonvl/docker-openvpn