fix(xanthous/server): Mount the key into the docker image

mount the host key in as a single-file volume, so the server can
actually read it.

Change-Id: I5fb58536717c91480d1f4610b6fb3258a36169e0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3864
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/server/module.nix

@@ -41,7 +41,8 @@ in {
         "${toString cfg.port}:22"
         "${toString cfg.metricsPort}:9000"
       ];
-      environment.SECRET_KEY_FILE = "/etc/secrets/xanthous-server-secret-key";
+      environment.SECRET_KEY_FILE = "/secret-key";
+      volumes = [ "/etc/secrets/xanthous-server-secret-key:/secret-key" ];
     };
   };
 }