nginx + phoenix configuration to server phoenix behind nginx at path 'phoenix'. nginx rewrites incoming URLs to strip the /phoenix, while the config.exs for phoenix ensures URLs are prefixed with /phoenix using the config.url.path

config.exs

# This file is responsible for configuring your application
# and its dependencies with the aid of the Mix.Config module.
#
# This configuration file is loaded before any dependency and
# is restricted to this project.
use Mix.Config

# Configures the namespace used by Phoenix generators
config :my_app,
  app_namespace: MyApp

# Configures the endpoint
config :my_app, MyApp.Endpoint,
  url: [host: "localhost", path: "/phoenix"],

nginx.conf

http {
  upstream phoenix_upstream {
    server 127.0.0.1:4000;
  }
  
  server {
    location /phoenix {
      proxy_redirect    off;

      rewrite /phoenix(.*) /$1 break;
      proxy_pass http://phoenix_upstream;
    }
  }
}