[docker] use go modules

Author: mdevan

Dockerfile

@@ -1,9 +1,8 @@
 FROM golang:1.11
-RUN go get -d -v github.com/rapidloop/pgmetrics
-RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o /root/pgmetrics github.com/rapidloop/pgmetrics/cmd/pgmetrics
+RUN GO111MODULES=on CGO_ENABLED=0 GOOS=linux go get -a -installsuffix cgo github.com/rapidloop/pgmetrics/cmd/pgmetrics
 
 FROM alpine:latest  
 RUN apk --no-cache add ca-certificates
-COPY --from=0 /root/pgmetrics /bin/pgmetrics
+COPY --from=0 /go/bin/pgmetrics /bin/pgmetrics
 ENTRYPOINT ["/bin/pgmetrics"]