Web services providing Continous Integration (CI) have been around for several years. Many of them like Circle CI, Cirrus CI and Travis CI offer free compute time for open source projects. At the beginning, only Linux was supported but with growing popularity support for additional operating systems has been added.
Cirrus CI is the first continuous integration system that supports FreeBSD in addition to Linux and Windows containers and macOS VMs. It is free of charge for open source projects and can easily be implemented by including a .cirrus.yml file. Multiple FreeBSD releases (amd64) are currently supported including 12.0-RELEASE. More information in the Cirrus CI guide: FreeBSD VMs.