Feature Request: Support beta releases

[melithine]

The problems with the switch to the Xabe wrapper around ffmpeg point out that having a beta system would be helpful in situations like these. Our dev team is small and time constrained, so leveraging users to help do some beta testing of larger changes could be quite helpful. I'm not exactly sure how GitHub handles this from the release side of things and what we would want to do with regards to the Docker images, given most Docker users just use the :latest tag instead of a specific version number.