using multiple stage build for alpine. (#279)

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

Dockerfile

@@ -1,8 +1,22 @@
-FROM centurylink/ca-certs
-EXPOSE 8088
+FROM alpine:3.6 as alpine
+RUN apk add -U --no-cache ca-certificates
+
+FROM scratch
+MAINTAINER Bo-Yi Wi <appleboy.tw@gmail.com>
+
+ENV GODEBUG=netdns=go
+
+COPY --from=alpine /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+
+LABEL org.label-schema.version=latest
+LABEL org.label-schema.vcs-url="https://github.com/appleboy/gorush.git"
+LABEL org.label-schema.name="Gorush"
+LABEL org.label-schema.vendor="Bo-Yi Wu"
+LABEL org.label-schema.schema-version="1.0"
 
 ADD config/config.yml /
 ADD bin/gorush /
 
+EXPOSE 8088
 ENTRYPOINT ["/gorush"]
 CMD ["-c", "config.yml"]