Update the Docker support for Review Board 4.0.
Review Request #11488 — Created Feb. 25, 2021 and submitted
This updates the brand-new Docker support to work with Review Board 4.0.
Mainly, this updates some of the package names, sets up for pre-release
builds, and switches to Python 3.
Built a new image and deployed a container. Saw that it successfully
installed and ran Review Board.
Tested upgrades on existing site directories.
Tested that Power Pack (using an in-progress Python 3 build) and
Review Bot (same) were both installed and working.