Enable hydra on weasel

m/weasel/configuration.nix

@@ -5,6 +5,7 @@
     ../common/ssf.nix
     ../module/hut-substituter.nix
     ./virtualization.nix
+    ./hydra.nix
   ];
 
   # Select this using the ID to avoid mismatches

m/weasel/hydra.nix

@@ -0,0 +1,13 @@
+{ config, ... }:
+{
+  services.hydra = {
+    enable = true;
+    hydraURL = "http://localhost:3001"; # externally visible URL
+    notificationSender = "hydra@jungle.bsc.es"; # e-mail of Hydra service
+    port = 3001;
+    # a standalone Hydra will require you to unset the buildMachinesFiles list to avoid using a nonexistant /etc/nix/machines
+    buildMachinesFiles = [ ];
+    # you will probably also want, otherwise *everything* will be built from scratch
+    useSubstitutes = true;
+  };
+}