The versions have to be stored as annotated git tags in the format of semantic versioning.
To create a new annotated release tag:
git tag -a 1.0.0-alpha.1 -m "new alpha release of version 1.0.0"
git push --tagsFollowing commits without a release tag will have the snapshotSuffix (default SNAPSHOT) appended
and the version number bumped according to incrementer (default minor) strategy, e.g., 1.1.0-alpha.1-SNAPSHOT.