docker-proxy: new package

Add docker-proxy package to Buildroot. Needed by docker-engine at
runtime, and previously not included. This leads to the following error
when attempting to port-map ports to a container:

  $ docker run -p 8080:8080 nginx
  docker: Error response from daemon: driver failed programming external
  connectivity on endpoint:
  exec: "docker-proxy": executable file not found in $PATH.

Docker expects the docker-proxy binary to exist in the PATH.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>