guests/coreos: fix test for docker daemon running

CHANGELOG.md

@@ -3,6 +3,7 @@
 BUG FIXES:
 
   - command/rsync-auto: Works properly on Windows.
+  - guests/coreos: Fix test for Docker daemon running.
   - guests/linux: Fix test for Docker provisioner whether Docker is
       running.
 

plugins/guests/coreos/cap/docker.rb

@@ -3,7 +3,7 @@ module VagrantPlugins
     module Cap
       module Docker
         def self.docker_daemon_running(machine)
-          machine.communicate.test("test -f /run/docker.sock")
+          machine.communicate.test("test -S /run/docker.sock")
         end
       end
     end