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.