This removes the need for the validation workaround for Docker, because box_updated is once again available in that context. We don't technically need the SyncedFolder priorities back on the Ruby side, but wiring them through for symmetry. Depends on https://github.com/hashicorp/vagrant-plugin-sdk/pull/183
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.
Generate proto
grpc_tools_ruby_protoc -I . --ruby_out=gen/plugin --grpc_out=gen/plugin ./plugin_server.proto