def _imgref_for_invocation_digest(build_context_digest):
    """
    Returns the image reference to tag an image given the digest of its build
    context.

    Such an image reference allows linking together a build context and
    an image.

    Args:
        build_context_digest: The hex digest for the build context.
    Returns:
        An image reference.
    """
    return 'build-context:' + build_context_digest