add Dockerfile

Dockerfile

+FROM golang:1.8-alpine
+ADD . /go/src/github.com/kumina/dovecot_exporter
+WORKDIR /go/src/github.com/kumina/dovecot_exporter
+RUN apk add --no-cache git && \
+    go get -v ./... && \
+    go build
+
+FROM alpine:latest
+EXPOSE 9166
+WORKDIR /
+COPY --from=0 /go/src/github.com/kumina/dovecot_exporter/dovecot_exporter .
+ENTRYPOINT ["/dovecot_exporter"]