It’s the first COPY command that isn’t cached:
remote: Step 1/27 : FROM elixir:1.9.0-alpine as build
remote: ---> 7a6d28e4b511
remote: Step 2/27 : RUN apk add --update git build-base
remote: ---> Using cache
remote: ---> 11da61788331
remote: Step 3/27 : RUN mkdir /app
remote: ---> Using cache
remote: ---> 2a0fd2bc9f5c
remote: Step 4/27 : WORKDIR /app
remote: ---> Using cache
remote: ---> 876adb368066
remote: Step 5/27 : RUN mix local.hex --force && mix local.rebar --force
remote: ---> Using cache
remote: ---> eb75832955a0
remote: Step 6/27 : ENV MIX_ENV=prod
remote: ---> Using cache
remote: ---> 3bc485b4dc82
remote: Step 7/27 : COPY mix.exs ./
remote: ---> 2561f54cdafa
<snip>
This seems to be an issue with Dokku, and not Elixir, so I’m continuing debugging there: Dockerfile builds only caching up to first COPY · Issue #3631 · dokku/dokku · GitHub
