🐛 Bug Report

If you deploy to GitHub pages with a custom domain, the custom domain will be removed from the repository settings and you need to re-add it after every deploy.

Have you read the Contributing Guidelines on issues?

Yes I did.

To Reproduce

1. In your GitHub repository settings,
   configure a custom domain for GitHub pages
2. In docusaurus.config.js, set url to your custom domain, with / as baseUrl
3. Follow the Deployment guide for GitHub pages
4. Run yarn deploy

Expected behavior

It should deploy the site at the custom domain and not modify the repository settings

Actual Behavior

It deploys the site at the default github.io domain, if you go back to the GitHub repository settings you can notice the custom domain was removed, you need to manually re-add it.

Your Environment

Docusaurus v2.0.0-alpha.70

Reproducible Demo

https://github.com/Alex1304/botrino