Update Dockerfile to use the stable release of kubectl.
The last build used latest at version v1.7.0-alpha-3 per README Current stable is v1.8.4 Current latest is v1.10.0-alpha.0, which outputs a breaks gitlab-registry secret .dockercfg format.
The last build used latest at version v1.7.0-alpha-3 per README Current stable is v1.8.4 Current latest is v1.10.0-alpha.0, which outputs a breaks gitlab-registry secret .dockercfg format.