add nginx-golang-postgres application sample

Signed-off-by: Anca Iordache <anca.iordache@docker.com>

samples/nginx-golang-postgres/backend/Dockerfile

@@ -0,0 +1,10 @@
+FROM golang:1.13-alpine AS build
+WORKDIR /go/src/github.com/org/repo
+COPY . .
+
+RUN go build -o server .
+
+FROM alpine:3.7
+EXPOSE 8000
+COPY --from=build /go/src/github.com/org/repo/server /server
+CMD ["/server"]