Just attach a tty when running the docker server

If STDIN is not a tty, IRB will use :NULL as a :PROMPT_MODE. See
https://github.com/ruby/ruby/blob/trunk/lib/irb/init.rb#L107.

If you are in teh future and it changed, it was:

    @CONF[:PROMPT_MODE] = (STDIN.tty? ? :DEFAULT : :NULL)