Set up release line branches

Set up release line branches for releasing versions of the Docker container for older versions of Antora. These branches should match the names of the release line branches in the upstream (e.g., v1.1.x).

The policy this image follows is that there's exactly one version of the image for each released version of Antora. The greatest stable release is duplicated with the Docker tag "latest". The greatest prerelease is duplicated with the Docker tag "testing".

As part of this change, it will be necessary to ensure that no tag is pushed for a stable release of an older version (e.g., 1.1.2). The latest tag should only be pushed for the latest stable version and the testing tag only for a prerelease. Otherwise, no tag should be used.

When a release of an older version of Antora is made, it will trigger the CI build for the corresponding branch in this repository. So that branch has to be ready to release the associated Docker image.