v1.8.0 allow set some k8s config via env variable;

- allow set DOCKER_IMAGE_PREFIX and KUBECTL_CA_PATH via env variable
- bugfix: properly escape DOCKER_PASSWORD when running docker login
- bugfix: properly config cache for pip when running "make ci-build"
  it needs a dir at /.cache that the current user can write into, inside
  the "docker run" docker container.