Add runner.docker_host configuration

What does this merge request do and why?

Add runner.docker_host configuration

Useful override for users with non-standard docker hosts. For example, I'm using plain lima instead of colima (there is a good reason for that, but I don't remember what it is), and GDK's runner can't autodetect my docker host.

How to set up and validate locally

1. In gdk.yml, set something like:

   runner:
     enabled: true
     token: 'your token'
     docker_host: 'your custom docker host'

2. Run make gitlab-runner-config.toml
3. Observe that the file was generated with your host value in the [runners.docker] section

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.