Comment 1 for bug 1903476

Pull request here: user/sandbergja/lp1903476_circleci_docs_building

Some info about CircleCI's free tier for open source organizations: https://circleci.com/open-source/

To test the docker part only:

1) Cherry-pick this commit into your own github repository
that includes the antora documentation
2) Install docker and docker-compose
3) Run this command: `docker-compose up -d`
4) Notice that the documentation is generated into the
docs-compiled directory

To test the whole integration:

1) In CircleCI, "follow" your github repository that
includes the antora documentation and this commit
2) In your CircleCI Project Settings, go to
Environment Variables.
3) Enter your Circle CI API Token as CIRCLE_TOKEN,
and Github API Token as GITHUB_TOKEN
4) In the `Advanced Settings` page of your project's
settings, enable `Only build pull requests`
5) Open a pull request that makes a change to the
antora documentation.
6) Wait for the "All checks have passed" notification.
Click "Show all checks". One of them should be a link
to your documentation.
7) Ideally, there will also be a comment automatically
added to your pull request.