Make the webhook endpoint URL configurable

In some CI setups, the URL to access the CI server differs from the
URL that the (external) webhook endpoint listens on. This change
introduces an option webhookBaseUrl that allows setting that URL
independently from the buildbot installation URL.

Author: antifuchs

nix/master.nix

@@ -181,6 +181,13 @@ in
         example = "buildbot.numtide.com";
       };
 
+      webhookBaseUrl = lib.mkOption {
+        type = lib.types.str;
+        description = "URL base for the webhook endpoint that will be registered for github or gitea repos.";
+        example = "https://buildbot-webhooks.numtide.com/";
+        default = "${config.services.buildbot-master.buildbotUrl}";
+      };
+
       outputsPath = lib.mkOption {
         type = lib.types.nullOr lib.types.path;
         description = "Path where we store the latest build store paths names for nix attributes as text files. This path will be exposed via nginx at \${domain}/nix-outputs";
@@ -272,7 +279,7 @@ in
               )"
               },
               admins=${builtins.toJSON cfg.admins},
-              url=${builtins.toJSON config.services.buildbot-master.buildbotUrl},
+              url=${builtins.toJSON config.services.buildbot-nix.master.webhookBaseUrl},
               nix_eval_max_memory_size=${builtins.toJSON cfg.evalMaxMemorySize},
               nix_eval_worker_count=${
                 if cfg.evalWorkerCount == null then "None" else builtins.toString cfg.evalWorkerCount