- use specific docker version 19.03.1 in example

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.

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues for most docs in
/ci, and cleans up other minor whitespace issues too.

`docker-in-docker` is a workflow not an specific executor. It uses the
[Docker executor](https://docs.gitlab.com/runner/executors/docker.html).

Also make security warning more visible and
improves instructions.

fix docs typo: register runner with services

See merge request gitlab-org/gitlab-ce!30769

Add usage of `default:`

Closes #63351

See merge request gitlab-org/gitlab-ce!30677