Fix unresolvable DOCKER_HOST by setting environment variable

Container registry tests creating a docker client fail with:

docker.errors.DockerException: Error while fetching server API version: HTTPSConnectionPool(host='docker', port=2376): Max retries exceeded with url: /version (Caused by NameResolutionError("<urllib3.connection.HTTPSConnection object at 0x7f5f08689490>: Failed to resolve 'docker' ([Errno -2] Name does not resolve)"))