Add multi-stage build of docker edge image (#7005)

This is to reduce the overall size of the image,
we only retain the binary that was built in previous stage.

Dockerfile

@@ -1,28 +1,32 @@
-FROM golang:1.10.1-alpine3.7
+FROM golang:1.10.4-alpine3.7
 
 LABEL maintainer="Minio Inc <dev@minio.io>"
 
 ENV GOPATH /go
-ENV PATH $PATH:$GOPATH/bin
 ENV CGO_ENABLED 0
+
+WORKDIR /go/src/github.com/minio/
+
+RUN  \
+     apk add --no-cache git && \
+     go get -v -d github.com/minio/minio && \
+     cd /go/src/github.com/minio/minio && \
+     go install -v -ldflags "$(go run buildscripts/gen-ldflags.go)"
+
+FROM alpine:3.7
+
 ENV MINIO_UPDATE off
 ENV MINIO_ACCESS_KEY_FILE=access_key \
     MINIO_SECRET_KEY_FILE=secret_key
 
-WORKDIR /go/src/github.com/minio/
+EXPOSE 9000
 
+COPY --from=0 /go/bin/minio /usr/bin/minio
 COPY dockerscripts/docker-entrypoint.sh dockerscripts/healthcheck.sh /usr/bin/
 
 RUN  \
      apk add --no-cache ca-certificates 'curl>7.61.0' && \
-     apk add --no-cache --virtual .build-deps git && \
+     echo 'hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4' >> /etc/nsswitch.conf
-     echo 'hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4' >> /etc/nsswitch.conf && \
-     go get -v -d github.com/minio/minio && \
-     cd /go/src/github.com/minio/minio && \
-     go install -v -ldflags "$(go run buildscripts/gen-ldflags.go)" && \
-     rm -rf /go/pkg /go/src /usr/local/go && apk del .build-deps
-
-EXPOSE 9000
 
 ENTRYPOINT ["/usr/bin/docker-entrypoint.sh"]