Improve entrypoint in docker container
Improve entrypoint in docker container

This commit switches from the "shell" form to the "exec" form of the
ENTRYPOINT statement. As a result, handing options upon starting the
capwap-dp container actually works. Example given:

    $> docker run -v /dp.conf:/dp.conf:ro capwap-dp -c /dp.conf

travelping/capwap-dp,travelping/capwap-dp