added dockerfile

Dockerfile

@@ -0,0 +1,16 @@
+FROM golang:1.9-alpine3.6 AS builder
+WORKDIR /go/src/github.com/caarlos0/domain_exporter
+ADD . .
+RUN apk add -U git
+RUN go get -v github.com/golang/dep/...
+RUN dep ensure -v
+ENV CGO_ENABLED=0
+ENV GOOS=linux
+ENV GOARCH=amd64
+RUN go build --ldflags "-extldflags "-static"" -o domain_exporter .
+
+FROM scratch
+EXPOSE 9222
+WORKDIR /
+COPY --from=builder /go/src/github.com/caarlos0/domain_exporter/domain_exporter .
+ENTRYPOINT ["./domain_exporter"]