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I managed to solve this one myself.  For anyone else with the same issue, instead of using a string for Build which is set to the directory containing the Dockerfile, instead use the following:

Build = new Docker.DockerBuild()
{
    Context = "../relative_path_of_directory_you_want_to_be_provided_to_the_dockerfile",
    Dockerfile = "../path_to_docker_directory/Dockerfile"
}