Added Makefile and updated Dockerfile

The Dockerfile is now more like the weave one, with a scratch layer
underneath.

Makefile taken more or less from weave, too; except, it uses
environment variables to cross-compile the binary (which should
probably be done for weave as well)

dns/Dockerfile

@@ -1,3 +1,5 @@
-FROM ubuntu
-EXPOSE 53/udp
-ADD ./dns /home/dns
+FROM scratch
+# EXPOSE 53/udp
+WORKDIR /home/weave
+ADD ./dns /home/weave/weavedns
+ENTRYPOINT ["/home/weave/weavedns"]

dns/Makefile

@@ -0,0 +1,20 @@
+IMAGE:=/tmp/weavedns.tar
+
+.DEFAULT: docker-image
+
+all: docker-image
+
+dns: ../*.go main.go
+	go get && GOARCH=amd64 GOOS=linux go build -v
+
+.PHONY: docker-image
+
+docker-image: $(IMAGE)
+
+$(IMAGE): Dockerfile dns
+	sudo -E docker build --rm -t zettio/weavedns .
+	sudo -E docker save zettio/weavedns > $(IMAGE)
+
+clean:
+	-sudo docker rmi zettio/weavedns
+	rm -f dns $(IMAGE)