xcaddy: init at 0.3.1.

A tool that can be used to build the Caddy webserver with plugins.

It functions as its own package manager and calls out to the network, so
it can't build Caddy in nixpkgs, but it's useful for environments that:

1. Do not need sandboxed and reproducible builds.

2. Pin Caddy and plugin versions when using the tool and trust it to be
   a reproducible, fixed derivation.

pkgs/servers/caddy/xcaddy/default.nix

@@ -0,0 +1,24 @@
+{ lib, buildGoModule, fetchFromGitHub }:
+
+buildGoModule rec {
+  pname = "xcaddy";
+  version = "0.3.1";
+
+  subPackages = [ "cmd/xcaddy" ];
+
+  src = fetchFromGitHub {
+    owner = "caddyserver";
+    repo = pname;
+    rev = "v${version}";
+    hash = "sha256-oGTtS5UlEebIqv4SM4q0YclASJNu8DNOLrGLRRAtkd8=";
+  };
+
+  vendorHash = "sha256-RpbnoXyTrqGOI7DpgkO+J47P17T4QCVvM1CfS6kRO9Y=";
+
+  meta = with lib; {
+    homepage = "https://github.com/caddyserver/xcaddy";
+    description = "Build Caddy with plugins";
+    license = licenses.asl20;
+    maintainers = with maintainers; [ tjni ];
+  };
+}

pkgs/top-level/all-packages.nix

@@ -3221,6 +3221,8 @@ with pkgs;
 
   caddy = callPackage ../servers/caddy { };
 
+  xcaddy = callPackage ../servers/caddy/xcaddy { };
+
   traefik = callPackage ../servers/traefik { };
 
   traefik-certs-dumper = callPackage ../tools/misc/traefik-certs-dumper { };