CircleCI fails with local registry name

[ryanlovett]

CircleCI tests are failing on

=========================== short test summary info ============================
FAILED tests/test_imagebuilder.py::test_build_image - docker.errors.DockerException: invalid tag 'localhost:60277/test-build-image:27a2e36': invalid reference format
FAILED tests/test_imagebuilder.py::test_parent_image_fetching - docker.errors.DockerException: invalid tag 'localhost:44897/parent-image-fetching:703b552': invalid reference format
========================= 2 failed, 1 passed in 6.64s ==========================

The image name is of the form {registry}/{image}:{tag}, however since the tests use localhost:{port} as the registry, docker.py doesn't like the image name.