update dockerfile to allow use of https_google - fixes #738 (#739)

* update dockerfile to allow use of https_google

to use TLS in a docker container (based on Alpine linux) you need to also include the CA certificate files

* cleaned up version of Dockerfile that supports https_google

Dockerfile

 FROM alpine:latest
 MAINTAINER Miek Gieben <miek@miek.nl> @miekg
 
-RUN apk --update add bind-tools && rm -rf /var/cache/apk/*
+# only need ca-certificates & openssl if want to use https_google
+RUN apk --update add bind-tools ca-certificates openssl && update-ca-certificates && rm -rf /var/cache/apk/*
 
 ADD coredns /coredns