Add Dockerfile (#116)

Add some docker bits and a Makefile.

Dockerfile

@@ -0,0 +1,9 @@
+FROM alpine:latest
+MAINTAINER Miek Gieben <miek@miek.nl> @miekg
+
+RUN apk --update add bind-tools && rm -rf /var/cache/apk/*
+
+ADD coredns /coredns
+
+EXPOSE 53 53/udp
+ENTRYPOINT ["/coredns"]

Makefile

@@ -0,0 +1,7 @@
+all:
+	go build
+
+.PHONY: docker
+docker:
+	GOOS=linux go build -a -tags netgo -installsuffix netgo
+	docker build -t $$USER/coredns .