New "Flakes" chapter

This also includes two other changes:

- Upgrade the Nix / Nixpkgs version
- Lock the flake files

Unfortunately it will be a bit tricky to decouple those changes so I'm
bundling them together.

Author: Gabriella439

manuscript/AdvancedModules.md

@@ -36,7 +36,7 @@ I will make use of this trick in a few examples below, so that we can simulate m
 
 ## `lib` utilities
 
-Nixpkgs provides several utility functions for NixOS modules that are stored underneath the "`lib`" hierarchy, and you can find the source code for those functions in [`lib/modules.nix`](https://github.com/NixOS/nixpkgs/blob/22.05/lib/modules.nix).
+Nixpkgs provides several utility functions for NixOS modules that are stored underneath the "`lib`" hierarchy, and you can find the source code for those functions in [`lib/modules.nix`](https://github.com/NixOS/nixpkgs/blob/23.11/lib/modules.nix).
 
 {blurb, class: information}
 If you want to become a NixOS module system expert, take the time to read and understand all of the code in `lib/modules.nix`.
@@ -318,7 +318,7 @@ error: The option `systemd.services.nginx.serviceConfig.RestartSec' has conflict
 (use '--show-trace' to show detailed location information)
 ```
 
-The problem is that when we enable `nginx` that automatically defines a whole bunch of other NixOS options, [including `systemd.services.nginx.serviceConfig.RestartSec`](https://github.com/NixOS/nixpkgs/blob/nixos-22.05/nixos/modules/services/web-servers/nginx/default.nix#L890).  This option is a scalar string option that disallows multiple distinct values because the NixOS module system by default has no way to known which one to pick to resolve the conflict.
+The problem is that when we enable `nginx` that automatically defines a whole bunch of other NixOS options, [including `systemd.services.nginx.serviceConfig.RestartSec`](https://github.com/NixOS/nixpkgs/blob/23.11/nixos/modules/services/web-servers/nginx/default.nix#L1234).  This option is a scalar string option that disallows multiple distinct values because the NixOS module system by default has no way to known which one to pick to resolve the conflict.
 
 However, we can use `mkOverride` to annotate our value with a higher priority so that it overrides the other conflicting definition:
 
@@ -472,7 +472,7 @@ If you launch the above NixOS configuration you should be able to verify that th
 [root@nixos:~]# systemctl status cowsay
 ○ cowsay.service
      Loaded: loaded (/etc/systemd/system/cowsay.service; enabled; preset: enabl>
-     Active: inactive (dead) since Sat 2022-11-05 20:11:05 UTC; 43s ago
+     Active: inactive (dead) since Sat 2023-11-05 20:11:05 UTC; 43s ago
    Duration: 106ms
     Process: 683 ExecStart=/nix/store/v02wsh00gi1vcblpcl8p103qhlpkaifb-unit-scr>
    Main PID: 683 (code=exited, status=0/SUCCESS)

manuscript/Book.txt

@@ -7,3 +7,4 @@ Modules.md
 AdvancedModules.md
 Terraform.md
 ContinuousIntegrationDeployment.md
+Flakes.md

manuscript/ContinuousIntegrationDeployment.md

@@ -131,7 +131,7 @@ You can use `nix repl` to explore flake outputs by running:
 
 ```bash
 $ nix repl .#
-Welcome to Nix 2.11.0. Type :? for help.
+Welcome to Nix 2.18.1. Type :? for help.
 
 Loading installable 'path:/Users/gabriella/proj/todo-app#'...
 Added 1 variables.
@@ -336,7 +336,7 @@ So `sops` doesn't completely solve the problem of secrets management (you still
 To use the `sops` command-line tool we'll need to convert our SSH primary key pair into an age key pair.  This step is performed by the admin who has access to both the SSH public key and the SSH private key and requires the `ssh-to-age` command-line tool, which you can obtain like this:
 
 ```bash
-$ nix shell 'github:NixOS/nixpkgs/22.11#ssh-to-age'
+$ nix shell 'github:NixOS/nixpkgs/23.11#ssh-to-age'
 ```
 
 The public key of our age key pair will be stored in a `.sops.yaml` configuration file which lives in version control.  To create the age public key, run:
@@ -380,7 +380,7 @@ Create a [fine-grained personal access token](https://docs.github.com/en/authent
 Fetch the `sops` command-line tool by running:
 
 ```bash
-$ nix shell 'github:NixOS/nixpkgs/22.11#sops'
+$ nix shell 'github:NixOS/nixpkgs/23.11#sops'
 ```
 
 … and then create a new secrets file by running:
@@ -461,7 +461,7 @@ Now we can distribute the GitHub personal access token stored inside of `secrets
 
 ```nix
 { inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/22.11";
+    nixpkgs.url = "github:NixOS/nixpkgs/23.11";
 
     sops-nix.url = "github:Mic92/sops-nix/bd695cc4d0a5e1bead703cc1bec5fa3094820a81";
   };