Implement initial containers module

nixos/modules/secrets.nix

@@ -8,6 +8,7 @@ let
 in
 {
   options.my.secrets = with lib.types; {
+    vmKeyPath = mkOpt' str "/tmp/xchg/dev.key" "Path to dev key when in a dev VM.";
     key = mkOpt' (nullOr str) null "Public key that secrets for this system should be encrypted for.";
     files = mkOpt' (attrsOf unspecified) { } "Secrets to decrypt with agenix.";
   };
@@ -19,7 +20,7 @@ in
       } // opts) cfg.files;
     }
     (mkIf config.my.build.isDevVM {
-      age.identityPaths = [ "/tmp/xchg/dev.key" ];
+      age.identityPaths = [ cfg.vmKeyPath ];
     })
   ];
 }