Docker build confd and package into alpine

Dockerfile

FROM golang:1.8.3-alpine3.6 AS confd-build
WORKDIR /go/src/github.com/kelseyhightower
RUN apk --update add git bash
RUN git clone --depth 1 https://github.com/kelseyhightower/confd.git \
    && cd confd \
    && ./build

FROM alpine:3.6
COPY --from=confd-build /go/src/github.com/kelseyhightower/confd/bin/confd /bin/
RUN chmod +x /bin/confd
WORKDIR /etc/confd
RUN mkdir conf.d templates
VOLUME /etc/confd
ENTRYPOINT ["confd"]