When buildkit is enabled docker will write out `writing image <image id>` When buildkit is not enables docker will write out `Successfully builld <image id>` It is more likely that searching for the `writing image` will not clash with build output than when searching for `Successfully built`. eg. when installing python packages with pip, it is common to use the verbage `Successfully built`.
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.