exposing the docker socket and binary into the toolbox container

This fixes the issue #8.

The patch creates the docker wrapper script to run `/run/media/usr/bin/docker` outside the container and bind-mounts it together with the socket only when the -d option is given to toolbox. This way when run later without -d option `toolbox` would not pollute the container with a wrapper script. Similarly, even if the container already has docker binary, `toolbox -d` hides that binary.