fix Dockerfile

Dockerfile

@@ -1,11 +1,17 @@
-# build stage
-FROM golang:1.9.3-alpine3.7
+FROM golang:1.9.3-alpine3.7 as backend
+RUN apk update && apk add ca-certificates curl git make tzdata
 
-ADD . /go/src/github.com/banzaicloud/spot-price-exporter
+RUN mkdir -p /go/src/github.com/banzaicloud/spot-price-exporter
+ADD Gopkg.* Makefile /go/src/github.com/banzaicloud/spot-price-exporter/
 WORKDIR /go/src/github.com/banzaicloud/spot-price-exporter
+
+RUN make vendor
+ADD . /go/src/github.com/banzaicloud/spot-price-exporter
+
 RUN go build -o /bin/spot-price-exporter .
 
-FROM alpine:latest
-RUN apk update && apk add ca-certificates && rm -rf /var/cache/apk/*
+
+FROM alpine:3.7
+COPY --from=backend /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
 COPY --from=0 /bin/spot-price-exporter /bin
 ENTRYPOINT ["/bin/spot-price-exporter"]