Look for docker on the $PATH
Look for docker on the $PATH

For example, on OSX a typical path is /usr/local/bin/docker

Closes #18

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

robhoes/xenserver-build-env,robhoes/xenserver-build-env,lindig/xs-shipyard