Rewrite of the Guestbook example in Go

This is a Go version of the Guestbook example. The readme has been ported over from the existing one.

#1271
You should add quotes around the variable or the full expression including the variable.

It might be cleaner if you use an auxiliary variable to name it, maybe ${guestbook_version} ?

```shell
guestbook_version=${1:-latest}
docker push "gurpartap/guestbook-example:${guestbook_version}"
```