Add Dockerfile for automatic build in DockerHub (#796)

* Add Dockerfile for automatic build in DockerHub
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Use build hooks to build docker images with SOURCE_BRANCH
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docker/Dockerfile

+FROM alpine:latest
+MAINTAINER Miek Gieben <miek@coredns.io> @miekg
+
+# 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/*
+
+ARG COREDNS_VERSION=latest
+
+RUN echo build coredns v$COREDNS_VERSION
+
+ADD https://github.com/coredns/coredns/releases/download/v${COREDNS_VERSION}/coredns_${COREDNS_VERSION}_linux_x86_64.tgz /
+
+RUN tar -xzf coredns_${COREDNS_VERSION}_linux_x86_64.tgz && rm coredns_${COREDNS_VERSION}_linux_x86_64.tgz
+
+EXPOSE 53 53/udp
+ENTRYPOINT ["/coredns"]

docker/hooks/build

+#!/bin/sh
+set -x
+docker build --build-arg COREDNS_VERSION=$SOURCE_BRANCH -t $IMAGE_NAME .