Bake release version at compiletime

LukasKnuth

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?

4 comments

#mix-release

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2025-08-12 15:42:56 UTC

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arcanemachine

Off the top of my head, I would try this:

ARG APP_VERSION
ENV APP_VERSION=$APP_VERSION

build arg != environment variable

EDIT: On second thought, maybe just skip the build arg part altogether and pass it as an environment variable:

docker build . -t my_app -e APP_VERSION=0.1.1

Then, I don’t think you would need to put anything in the Dockerfile.

Post #1

LostKobrakai

Not sure if that’s helpful, but all the places where things worked are the result of mix release, while Application.spec uses the .app file of the compiled application as created by mix compile. The .app file would be in lib/my_app-{version}/ebin/my_app.app of the release.

Post #2

LukasKnuth

Bingo! Even the folder is called lib/my_app-0.0.0-local/. I have moved the ARG APP_VERSION instruction higher in the Dockerfile, just below ENV MIX_ENV="prod" and now everything works as expected!

I wonder what step really creates the .app file, because I had the instruction right above mix compile before (where it didn’t work). After moving it, the following commands now also receive it:

mix deps.get
mix deps.compile
mix assets.deploy

None of which seems like an obvious candidate to me.

Post #3

rhcarvalho

One of those might be doing it as a side effect of their transitive dependencies. You don’t have to call “mix compile” exclusively to trigger compilation (and in that case your RUN mix compile line was likely a no-op.)

For example, it happens all the time when you run mix test – code is first compiled, then the test suite is run.

Post #4

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