I’m working on dockerizing a Phoenix app and I’m wondering if the MIX_ENV needs to be set as a Docker build-arg. Because environment variables are read at compile time, I think your Docker image would have to know about which MIX_ENV it should be using, right? In other words, when you build a Dockerfile for an Elixir/Phoenix app, you are building an environment-specific image (e.g. the prod build, the test build, etc).

I’m looking over this blog post https://revelry.co/elixir-releases/ and the Dockerfile it uses https://github.com/revelrylabs/revelry_phoenix_app_template/blob/master/Dockerfile – hard-codes the MIX_ENV right at the top of the file, but it seems to me that this could be replaced by a build-arg.

How are others handling this? A lot of CI could be simplified if you could build once and choose the environment later…