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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def container_def(image, model_data_url=None, env=None):
 """Create a definition for executing a container as part of a SageMaker model. Args: image (str):
 Docker image to run for this container. model_data_url (str):
 S3 URI of data required by this container, e.g. SageMaker training job model artifacts (default: None). env (dict[str, str]):
 Environment variables to set inside the container (default: None). Returns: dict[str, str]: A complete container definition object usable with the CreateModel API if passed via `PrimaryContainers` field. """

    if env is None:
        env = {}
    c_def = {'Image': image, 'Environment': env}
    if model_data_url:
        c_def['ModelDataUrl'] = model_data_url
    return c_def