Dockers use their own self contained OS's, they don't interact with the host OS like you are implying. see here. http://lime-technology.com/forum/index.php?topic=40937.msg387520#msg387520
If your docker needs access to a particular program, it needs to be installed inside the docker container, typically by docker exec into the container and installing from there. Installing the binary in unraid isn't going to help.
And if you install it yourself inside the docker, you will have to do it again each time you update the docker. Maybe you could get the author of the docker to include it for you.