Using agenix inside a declared container

If I have a container inside of my flake, similar to here:

I can’t use age.* inside the config = { } stanza, because it will error:

error: The option `containers.testcontainer.age' does not exist. Definition values:
       - In `/nix/store/mjwn8yvvd2j83srwbphsfpph6j3rkj58-source/nixos/modules/virtualisation/nixos-containers.nix':
             secrets = {
               pw_myusuer = {
                 file = /nix/store/4j7q6wfmmhjjjd1h2vi67sp7g8855asg-source/secrets/base/pw_myuser.age;

Is there a way to get agenix to work inside this definition?