Bake release version at compiletime

I’m using the standard Dockerfile generated by mix phx.gen.release --docker with a small change:

# Compile the release
ARG APP_VERSION
RUN mix compile

The added ARG expression accepts a Docker build-arg that I supply when building the image:

docker build . -t my_app --build-arg APP_VERSION=0.1.1

In my mix.exs I load the data from the Environment variable:

defmodule MyApp.MixProject do
  use Mix.Project

  def project do
    [ #Other config
      version: version()
    ]
  end

  def version do
    System.get_env("APP_VERSION", "0.0.0-local")
  end
end

I’m expecting that MyApp.MixProject.version/0 is invoked at compile time and that the ENV variable is read then and “baked” into the final release.
This seems to be true partially:

• during image build it logs: assembling my_app-0.1.1 on MIX_ENV=prod
• running bin/my_app version returns the expected 0.1.1
• the file releases/start_erl.data contains 15.2.1 0.1.1 (the later is the version)
• the folder in releases/0.1.1 exists

All this seems to indicate that it worked fine, but at runtime, i get the fallback value:

iex> Application.spec(:my_app, :vsn)
~c"0.0.0-local"

I can’t figure out why it seems to work everywhere except for the Application spec.

I have also tried setting the APP_VERSION env variable to some other string to see if it might be evaluated at runtime - it still returns "0.0.0-local". What am I overlooking?