nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/system/podman/default.nix

@@ -0,0 +1,48 @@
+# Podman related settings
+{ config, lib, ... }:
+let
+  cfg = config.my.system.podman;
+in
+{
+  options.my.system.podman = with lib; {
+    enable = mkEnableOption "podman configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = cfg.enable -> !config.my.system.docker.enable;
+        message = ''
+          `config.my.system.podman` is incompatible with
+          `config.my.system.docker`.
+        '';
+      }
+    ];
+
+    virtualisation.podman = {
+      enable = true;
+
+      # Use fake `docker` command to redirect to `podman`
+      dockerCompat = true;
+
+      # Expose a docker-like socket
+      dockerSocket.enable = true;
+
+      # Allow DNS resolution in the default network
+      defaultNetwork.settings = {
+        dns_enabled = true;
+      };
+
+      # Remove unused data on a weekly basis
+      autoPrune = {
+        enable = true;
+
+        dates = "weekly";
+
+        flags = [
+          "--all"
+        ];
+      };
+    };
+  };
+}