I’ve been working on many projects based on Phoenix in the last few years and worked on the transition from mix phx.server (for production environment) to OTP release with distillery then mix release. Starting last night, for a reason I can not explain, splitting mix deps.get and mix compile on different docker intermediate layers does not work anymore :

COPY mix.* ./
RUN mix deps.get --only ${MIX_ENV}

COPY . .
RUN mix compile

And fails with the following errors:

Unchecked dependencies for environment prod:
* parse_trans (Hex package)
  lock mismatch: the dependency is out of date. To fetch locked version run "mix deps.get"
* mimerl (Hex package)
  lock mismatch: the dependency is out of date. To fetch locked version run "mix deps.get"

…

* phoenix_ecto (Hex package)
  lock mismatch: the dependency is out of date. To fetch locked version run "mix deps.get"
** (Mix) Can't continue due to errors on dependencies
The command '/bin/sh -c mix compile' returned a non-zero code: 1

Moving RUN mix deps.get after COPY . . works, but it defies the caching purposes of isolating dependencies… I tried on both MacOS and Ubuntu 18.04 with the same result

We have been using this setup for as long as I can remember for every ruby/node/elixir projects we are packaging as Docker images. Take a look at the elixir-boilerplate project we are maintaining to start every web projects at Mirego.

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