providers/docker: properly copy create_args in config [GH-4526]

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 BUG FIXES:
 
+  - providers/docker: Create args works. [GH-4526]
   - providers/virtualbox: Show a human-friendly error if VirtualBox didn't
       clean up an existing VM. [GH-4681]
   - provisioners/docker: Search for docker binary in multiple places. [GH-4580]

plugins/providers/docker/config.rb

@@ -87,7 +87,7 @@ module VagrantPlugins
         @build_args = []
         @build_dir  = UNSET_VALUE
         @cmd        = UNSET_VALUE
-        @create_args = []
+        @create_args = UNSET_VALUE
         @env        = {}
         @expose     = []
         @force_host_vm = UNSET_VALUE