How to get a meaningful trace on a nixos-container build fail?

I am trying to create a container but the build fails with an error message which doesn’t help me much :

sudo nixos-container create bookwyrm --flake ./container/
host IP is, container IP is
warning: Git tree '/home/henri/travaux/nix_flakes/bookwyrm-flake' is dirty
       … while calling the 'head' builtin

         at /nix/store/hl93xdyddynkvgpvfpvyvy1qvw0jvf11-source/lib/attrsets.nix:816:11:

          815|         || pred here (elemAt values 1) (head values) then
          816|           head values
             |           ^
          817|         else

       … while evaluating the attribute 'value'

         at /nix/store/hl93xdyddynkvgpvfpvyvy1qvw0jvf11-source/lib/modules.nix:759:9:

          758|     in warnDeprecation opt //
          759|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          760|         inherit (res.defsFinal') highestPrio;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: cannot coerce null to a string
/run/current-system/sw/bin/nixos-container: failed to build container from flake './container/'
make: *** [Makefile:6: local] Error 2

The ‘/nix/store/hl93xdyddynkvgpvfpvyvy1qvw0jvf11-source/lib/modules.nix’ is from the nixos distribution.

If I try to add a “–show-trace” at the end of the command I get a Unknown option: show-trace (an issue about that is open since 2015 : nixos-container: wrong message for --show-trace · Issue #8970 · NixOS/nixpkgs · GitHub)

Do you have any idea how I can get more info ?