Fix inverted logic in Docker exploit

Positive condition should be tested first, imo. Confusing otherwise. My
bad, though.

Credit to @fslavin-r7.

Author: wvu

modules/exploits/linux/local/docker_daemon_privilege_escalation.rb

@@ -35,11 +35,11 @@ def initialize(info={})
   end
 
   def check
-    if cmd_exec('docker ps && echo true') == 'true'
+    if cmd_exec('docker ps && echo true') =~ /true$/
+      Exploit::CheckCode::Vulnerable
+    else
       print_error("Failed to access Docker daemon.")
       Exploit::CheckCode::Safe
-    else
-      Exploit::CheckCode::Vulnerable
     end
   end