Support Docker volume consistency for synced folders

Adds the `docker_consistency` option, which sets the Docker volume
consistency level. This can be used to greatly improved synced folder
performance, especially on macOS.

See for details: moby/moby#31047

plugins/providers/docker/synced_folder.rb

@@ -21,7 +21,10 @@ module VagrantPlugins
 
           host_path  = data[:hostpath]
           guest_path = data[:guestpath]
-          machine.provider_config.volumes << "#{host_path}:#{guest_path}"
+          # Append consistency option if it exists, otherwise let it nil out
+          consistency = data[:docker_consistency]
+          consistency &&= ":" + consistency
+          machine.provider_config.volumes << "#{host_path}:#{guest_path}#{consistency}"
         end
       end
     end

website/source/docs/docker/basics.html.md

@@ -75,6 +75,16 @@ on folders synced with a docker container.
 
 Private and public networks are not currently supported.
 
+### Volume Consistency
+
+Docker's [volume consistency](https://github.com/moby/moby/pull/31047) setting can be specified using the `docker_consistency` option when defining a synced folder. This can
+[greatly improve performance on macOS](https://docs.docker.com/docker-for-mac/osxfs-caching). An example is shown using the `cached` and `delegated` settings:
+
+```
+config.vm.synced_folder "/host/dir1", "/guest/dir1", docker_consistency: "cached"
+config.vm.synced_folder "/host/dir2", "/guest/dir2", docker_consistency: "delegated"
+```
+
 ## Host VM
 
 If the system cannot run Linux containers natively, Vagrant automatically spins