nix-repl> d = derivation { name = "foo"; builder = "${bash}/bin/bash"; args = [ ./builder.sh ]; system = builtins.currentSystem; }
nix-repl> :b d
[1 built, 0.0 MiB DL]
this derivation produced the following outputs:
out -> /nix/store/gczb4qrag22harvv693wwnflqy7lx5pb-foo