providers/docker: put sync folders in var lib docker [GH-3680]

CHANGELOG.md

@@ -19,6 +19,8 @@ BUG FIXES:
   - providers/docker: Show proper error message when on Linux. [GH-3654]
   - providers/docker: Proxy VM works properly even if default provider
       environmental variable set to "docker" [GH-3662]
+  - providers/docker: Put sync folders in `/var/lib/docker` because
+      it usually has disk space. [GH-3680]
   - synced\_folders/rsync: Create the directory before syncing.
 
 ## 1.6.0 (May 6, 2014)

plugins/providers/docker/action/host_machine_sync_folders.rb

@@ -111,7 +111,7 @@ module VagrantPlugins
               # If we specify exact then we know what we're doing
               if !data[:docker__exact]
                 data[:guestpath] =
-                  "/mnt/docker_#{Time.now.to_i}_#{rand(100000)}"
+                  "/var/lib/docker/docker_#{Time.now.to_i}_#{rand(100000)}"
               end
 
               # Add this synced folder onto the new config if we haven't