At least on macOS combo Catalina + Docker engine 19.03.4 + Docker desktop 2.1.0.4 + vagrant 2.2.6 the original `matches = result.scan(/Successfully built (.+)$/i)` -line fails to generate a match. With this change I can `vagrant up --provider=docker` successfully.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.