Use pre_build hook in docker hub to build the coredns binary (#799)

and skip the download process.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Makefile

@@ -8,7 +8,7 @@ all: coredns
 # TODO: Add .go file dependencies.
 .PHONY: coredns
 coredns: check godeps
-	go build $(BUILD_VERBOSE) -ldflags="-s -w"
+	CGO_ENABLED=0 go build $(BUILD_VERBOSE) -ldflags="-s -w"
 
 .PHONY: deps
 deps: core/zmiddleware.go core/dnsserver/zdirectives.go godeps

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 .

hooks/pre_build

+#!/bin/sh
+echo "=> Building the CoreDNS binary"
+docker run \
+  -v $(pwd):/go/src/github.com/coredns/coredns \
+  -w /go/src/github.com/coredns/coredns \
+  golang:1.8.0 \
+  make