using_docker_images.md: document services inheriting dns and addn hosts settings

What does this MR do?

This adds documentation about gitlab-runners service containers using the same dns servers, search domains and additional hosts as the main ci job container itself, which wasn't the case until gitlab-runner!1075 (merged)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.