xinydev: Update the instructions for build image in the readme

JobBuild
589
07-30 17:34
pull-npd-build 1432459916727029760
pull-npd-e2e-kubernetes-gce-gci 1432459916831887360 1421162260356665344
pull-npd-e2e-kubernetes-gce-gci-custom-flags 1432459916873830400 1421162260398608384
pull-npd-e2e-kubernetes-gce-ubuntu 1432459916903190528 1421162260448940032
pull-npd-e2e-kubernetes-gce-ubuntu-custom-flags 1432459916932550656 1421162260478300160
pull-npd-e2e-node 1432459916798332928
pull-npd-e2e-test 1432459916966105088
pull-npd-test 1432459916760584192