providers/docker: get rid of mutex around Docker access, should be safe

plugins/providers/docker/action/create.rb

@@ -4,7 +4,6 @@ module VagrantPlugins
       class Create
         def initialize(app, env)
           @app = app
-          @@mutex ||= Mutex.new
         end
 
         def call(env)
@@ -18,8 +17,6 @@ module VagrantPlugins
 
           params = create_params
 
-          cid = ''
-          @@mutex.synchronize do
           env[:ui].output(I18n.t("docker_provider.creating"))
           env[:ui].detail("  Name: #{params[:name]}")
           env[:ui].detail(" Image: #{params[:image]}")
@@ -34,7 +31,6 @@ module VagrantPlugins
           end
 
           cid = @driver.create(params)
-          end
 
           env[:ui].detail(" \n"+I18n.t(
             "docker_provider.created", id: cid[0...16]))