Add Attic binary cache and containerize gitea runner

Replace nix-serve-only setup with Attic for managed binary caching with
upstream filtering and GC. Move gitea actions runner from host into an
isolated NixOS container with private networking. nix-serve kept alongside
Attic during migration.

common/binary-cache.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ config, ... }:
 
 {
   nix = {
@@ -6,11 +6,11 @@
       substituters = [
         "https://cache.nixos.org/"
         "https://nix-community.cachix.org"
-        "http://s0.koi-bebop.ts.net:5000"
+        "http://s0.koi-bebop.ts.net:28338/nixos"
       ];
       trusted-public-keys = [
         "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
-        "s0.koi-bebop.ts.net:OjbzD86YjyJZpCp9RWaQKANaflcpKhtzBMNP8I2aPUU="
+        "nixos:SnTTQutdOJbAmxo6AQ3cbRt5w9f4byMXQODCieBH3PQ="
       ];
 
       # Allow substituters to be offline
@@ -19,6 +19,11 @@
       # and use this flag as intended for deciding if it should build missing
       # derivations locally. See https://github.com/NixOS/nix/issues/6901
       fallback = true;
+
+      # Authenticate to private nixos cache
+      netrc-file = config.age.secrets.attic-netrc.path;
     };
   };
+
+  age.secrets.attic-netrc.file = ../secrets/attic-netrc.age;
 }