If I run it without that flag, I get 

    Step 5 : RUN ln -s /bin/true /sbin/initctl
     ---> Running in 0a24bcd98b35
    ln: failed to create symbolic link `/sbin/initctl': File exists
    2014/03/24 07:57:27 The command [/bin/sh -c ln -s /bin/true /sbin/initctl] returned a non-zero code: 1