Improve docker build (#396)

Dockerfile

-FROM alpine:3.3
+FROM golang:1.12 AS builder
 
-RUN sed -i 's|dl-cdn.alpinelinux.org|mirrors.aliyun.com|g' /etc/apk/repositories
-RUN apk add --no-cache ca-certificates curl
+WORKDIR /work
+COPY . .
+RUN ls -hal && make linux
 
-RUN curl -L https://github.com/jenkins-zh/jenkins-cli/releases/latest/download/jcli-linux-amd64.tar.gz|tar xzv && \
-    mv jcli /usr/local/bin/
-
-ENTRYPOINT ["jcli"]
+FROM alpine:3.10
+COPY --from=builder /work/bin/linux/jcli /usr/bin/jcli