Fix some readme and script bugs in examples (take 2).

Technically this was an issue before this patch, but the variable access should be inside double quotes. Using ${...} also recommended.

```
docker build -t "${DOCKER_HUB_USER}/update-demo:kitten" images/kitten
```

Same for the other two lines below which refer to ${DOCKER_HUB_USER}.

Cheers!
Filipe