You should be using Application.app_dir(:app_name, "priv/dir/…") in your code to reference files in priv/ folder. You can use configuration for the second parameter if needed.

Additionally you can use the :code.priv_dir/1 function

FWIW these days I register these release-resiliant path concerns in my application’s compile-time config.exs. I find it to be more reliable and easier to reason about if I do it myself:

# config/config.exs
import Config

root_dir = Path.expand("../..", __DIR__)
priv_dir = Path.expand(Path.join(project_root, "priv"))
assets_dir = Path.expand(Path.join(project_root, "assets"))
static_dir = Path.expand(Path.join(priv_dir, "static"))
static_assets_dir = Path.expand(Path.join(static_dir, "assets"))

config :project, :root_dir, root_dir
config :project, :priv_dir, priv_dir
config :project, :assets_dir, assets_dir
config :project, :static_dir, static_dir
config :project, :static_assets_dir, static_assets_dir

# Use variables above to configure things like `:esbuild` in this file

# Use `Application.compile_env!(:project, :static_assets_dir)`
# to configure things like `Plug.Static`

This works well for umbrella apps, since the each have their own config :app_name space.

For often-referenced paths, I’ll even add a shortcut in my project’s main entrypoint module:

# lib/project/application.ex
defmodule Project.Application do
  def app_name, do: :project
  def root_dir, do: Application.compile_env!(app_name(), :root_dir)
  def static_dir, do: Path.relative_to(Application.compile_env!(app_name(), :static_dir), root_dir())
end

Then, for example, configuring Plug.Static can look like:

# lib/project_web/endpoint.ex
plug Plug.Static,
  at: "/",
  from: {Project.Application.name(), Project.Application.static_dir()}