Add reverse DNS info to prerequisites
What does this MR do and why?
This MR informs reader that he doesn't need to own a domain in order to selfhost GitLab.
- Reverse DNS can be used
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Screenshots or screen recordings
Screenshots are required for UI changes, and strongly recommended for all other merge requests.
| Before | After |
|---|---|
 |
 |
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
- Open doc/install/docker.md in any Markdown viewer