Allow different values for user_nsmode

create(...) currently allows only host as UsernsMode. This seems to make sense as docker only allows this as an option. However, podman supports a few others. I assume this package should be able to interact with other docker-compatible APIs, and in that case it would make sense that we can set this parameter differently.
Right now, the only workaround I've found was to use the low level API as such:

host_config = api_client.create_host_config(
    binds={
        socket_dir: {'bind': '/var/run/postgresql', 'mode': 'rw'},
        data_dir: {'bind': '/var/lib/postgresql/data', 'mode': 'rw'},
    }
)

# kind of hacky, right?
host_config['UsernsMode'] = 'keep-id'

container_config = api_client.create_container(
    image=POSTGRES_IMAGE,
    name=CONTAINER_NAME,
    user=f'{os.getuid()}:{os.getgid()}',
    host_config=host_config,
    environment={'POSTGRES_USER': 'postgres', 'POSTGRES_PASSWORD': 'postgres', 'POSTGRES_DB': db_name},
)

The issue names create(...) and the low-level create_host_config/create_container path; start there to find where UsernsMode is restricted. Check the Podman value keep-id and confirm that the higher-level path preserves it in the resulting host configuration, with coverage for the newly accepted value.