feat: Add back docker

Docker is actually useful for some things, and NixOS containers don't
replace pre-existing solutions with docker-compose files.

modules/default.nix

@@ -2,6 +2,7 @@
   imports = [
     ./boot.nix
     ./cdemu.nix
+    ./docker.nix
     ./dual-function-keys.nix
     ./fish.nix
     ./fonts.nix

modules/docker.nix

@@ -0,0 +1,9 @@
+{ pkgs, ... }:
+
+{
+  virtualisation.docker.enable = true;
+
+  environment.systemPackages = with pkgs; [
+    docker-compose
+  ];
+}