providers/docker: process-level lock on copying default Vagrantfile

lib/vagrant/environment.rb

@@ -373,7 +373,7 @@ module Vagrant
           # Mark that we have a lock
           @locks[name] = true
 
-          yield
+          return yield
         ensure
           # We need to make sure that no matter what this is always
           # reset to false so we don't think we have a lock when we

plugins/providers/docker/provider.rb

@@ -52,8 +52,17 @@ module VagrantPlugins
           # we don't write into our installation dir (we can't).
           default_path = File.expand_path("../hostmachine/Vagrantfile", __FILE__)
           vf_path      = @machine.env.data_dir.join("docker-host", "Vagrantfile")
+          begin
+            if !vf_path.file?
+              @machine.env.lock("docker-provider-hostvm") do
                 vf_path.dirname.mkpath
                 FileUtils.cp(default_path, vf_path)
+              end
+            end
+          rescue Vagrant::Errors::EnvironmentLockedError
+            # Lock contention, just retry
+            retry
+          end
 
           # Set the machine name since we hardcode that for the default
           host_machine_name = :default