Update DinD example for 19.03

Docker 19.03 enables TLS by default:

```
Starting in 18.09+, the dind variants of this image will automatically
generate TLS certificates in the directory specified by the
DOCKER_TLS_CERTDIR environment variable.

Warning: in 18.09, this behavior is disabled by default (for
compatibility). If you use --network=host, shared network namespaces (as
in Kubernetes pods), or otherwise have network access to the container
(including containers started within the dind instance via their gateway
interface), this is a potential security issue (which can lead to access
to the host system, for example). It is recommended to enable TLS by
setting the variable to an appropriate value (-e
DOCKER_TLS_CERTDIR=/certs or similar). In 19.03+, this behavior is
enabled by default.
```

Update the example to use docker over TLS.