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.

Dockerfile

@@ -21,7 +21,7 @@ ENV EASYRSA_VARS_FILE $OPENVPN/vars
 
 VOLUME ["/etc/openvpn"]
 
-# Internally uses port 1194, remap using docker
+# Internally uses port 1194/udp, remap using `docker run -p 443:1194/tcp`
 EXPOSE 1194/udp
 
 WORKDIR /etc/openvpn