add host variable

config/prod.exs

@@ -9,8 +9,14 @@ use Mix.Config
 # manifest is generated by the `mix phx.digest` task,
 # which you should run after static files are built and
 # before starting your production server.
+host =
+  System.get_env("HOST") ||
+    raise """
+    environment variable HOST is missing.
+    """
+
 config :pokemon_couture, PokemonCoutureWeb.Endpoint,
-  url: [host: "example.com", port: 80],
+  url: [host: host, port: 80],
   cache_static_manifest: "priv/static/cache_manifest.json"
 
 # Do not print debug messages in production