Force flag to prevent file already exists error.

I had been getting this error... 

    ln: failed to create symbolic link `/sbin/initctl': File exists
    2014/03/25 10:30:36 The command [/bin/sh -c ln -s /bin/true /sbin/initctl] returned a non-zero code: 1

Docker-DCO-1.1-Signed-off-by: No Ducks <onemannoducks@gmail.com> (github: noducks)