docs+nix: add deployment README and align release packaging

config/runtime.exs

@@ -0,0 +1,14 @@
+import Config
+
+if config_env() == :prod do
+  database_url =
+    System.get_env("DATABASE_URL") ||
+      raise "environment variable DATABASE_URL is missing. Example: ecto://USER:PASS@HOST/DATABASE"
+
+  config :parrhesia, Parrhesia.Repo,
+    url: database_url,
+    pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")
+
+  config :parrhesia, Parrhesia.Web.Endpoint,
+    port: String.to_integer(System.get_env("PORT") || "4000")
+end