From ALT Linux Wiki
hasher is a tool for building packages in safe and repeatable manner. All packages in Sisyphus repository are built using hasher.
hasher is similar to earlier ApplianceWare/ALT Linux sandman, Debian sbuild/pbuilder and also later Fedora mock, but unlike the mentioned tools it is designed and implemented with security in mind. hasher prevents interaction between package being built and host system and also between packages being built.
Copy of this documentation can be found in packages hasher and hasher-priv in Sisyphus repository.