[nix] Modularized the container backend so I can easily switch it with an option

system/hosts/mcp/containers/mariadb.nix

@@ -1,8 +1,8 @@
 # Common config for all mariadb containers
-{ ... }:
+{ config, ... }:
 {
   sops.secrets."mariadb_root_password" = {
-    restartUnits = [ "podman-mariadb.service" ];
+    restartUnits = [ "${config.local.container-backend}-mariadb.service" ];
     mode = "0440";
     group = "mariadb";
   };