Odd issue with error (ArgumentError) invalid :json_decoder option. The module Poison is not loaded and could not be found

Questions / Help
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1

I have a phoenix project (not an umbrella project) using Phoenix 1.6.16. I am able to compile and run this project locally without issues.

However when building as a docker image, I get the following build error.

== Compilation error in file lib/myapp_web/endpoint.ex ==
** (ArgumentError) invalid :json_decoder option. The module Poison is not loaded and could not be found
(plug 1.15.3) lib/plug/parsers/json.ex:48: Plug.Parsers.JSON.validate_decoder!/1
(plug 1.15.3) lib/plug/parsers/json.ex:35: Plug.Parsers.JSON.init/1
(plug 1.15.3) lib/plug/parsers.ex:282: anonymous fn/2 in Plug.Parsers.convert_parsers/2
(elixir 1.16.2) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
(elixir 1.16.2) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
(plug 1.15.3) lib/plug/parsers.ex:262: Plug.Parsers.init/1
(plug 1.15.3) lib/plug/builder.ex:341: Plug.Builder.init_module_plug/4
(plug 1.15.3) lib/plug/builder.ex:322: anonymous fn/5 in Plug.Builder.compile/3

If add the poison dependency, the build completes without errors.

I have the following in my config.exs

# Use Jason for JSON parsing in Phoenix
config :phoenix, :json_library, Jason

This only happens when building as a docker image. Building it locally on my mac has no build issues. Once I add the Poison dependency, the build succeeds without issues.

Docker base is Alpine 3.19 with Elixir 1.16.2 on erlang 25.

2

If I set the following in endpoint.ex it all works without having to add Poison as a dependency.

plug Plug.Parsers,
    parsers: [:urlencoded, :multipart, :json],
    pass: ["*/*"],
    json_decoder: Jason
3

NM, I am an idiot.

My Dockerfile was broken with an erroneous copy command for the config files. Fixed that and all is working normally.