use an algorithm to generate image paths in config

config/config.exs

@@ -54,8 +54,8 @@ config :phoenix, :json_library, Jason
 # Edit these values to describe your relay instance.
 config :gc_index_relay, :relay_info,
   name: "Mercury Index-Relay",
-  icon: "http://localhost:4000/favicon.ico",
-  banner: "http://localhost:4000/images/mercury_icon.png",
+  icon: "favicon.ico",
+  banner: "images/mercury_icon.png",
   description:
     "A Nostr index relay for the http protocol, from GitCitadel. Featuring a RESTful API and Swagger, it specializes in swift retrieval or publications, repos, and similar graphs of related events",
   software: "https://git.imwald.eu/silberengel/gc_index_relay.git",

lib/gc_index_relay_web/plugs/relay_info.ex

@@ -18,7 +18,12 @@ defmodule GcIndexRelayWeb.Plugs.RelayInfo do
     accept = conn |> get_req_header("accept") |> Enum.join(",")
 
     if String.contains?(accept, "application/nostr+json") do
-      relay_info = Application.get_env(:gc_index_relay, :relay_info, []) |> Map.new()
+      base_url = build_base_url(conn)
+
+      relay_info =
+        Application.get_env(:gc_index_relay, :relay_info, [])
+        |> Map.new()
+        |> resolve_image_urls(base_url)
 
       conn
       |> put_resp_content_type("application/nostr+json")
@@ -30,4 +35,39 @@ defmodule GcIndexRelayWeb.Plugs.RelayInfo do
   end
 
   def call(conn, _opts), do: conn
+
+  # Build "scheme://host[:port]" from the incoming request.
+  # Standard ports (80 for http, 443 for https) are omitted.
+  defp build_base_url(conn) do
+    port_suffix =
+      case {conn.scheme, conn.port} do
+        {:http, 80} -> ""
+        {:https, 443} -> ""
+        {_, port} -> ":#{port}"
+      end
+
+    "#{conn.scheme}://#{conn.host}#{port_suffix}"
+  end
+
+  # Prepend base_url to any relative (non-absolute) value for :icon and :banner.
+  defp resolve_image_urls(relay_info, base_url) do
+    relay_info
+    |> resolve_field(:icon, base_url)
+    |> resolve_field(:banner, base_url)
+  end
+
+  defp resolve_field(map, key, base_url) do
+    case Map.get(map, key) do
+      nil -> map
+      "" -> map
+      url when is_binary(url) ->
+        if String.starts_with?(url, ["http://", "https://"]) do
+          map
+        else
+          path = if String.starts_with?(url, "/"), do: url, else: "/#{url}"
+          Map.put(map, key, "#{base_url}#{path}")
+        end
+      _ -> map
+    end
+  end
 end