cached_build.sh

#!/bin/sh -eu
rm -rf .depscache
mkdir .depscache
rsync -r --exclude=.depscache . .depscache
pushd .depscache
find . -type f ! -wholename '*mix.*' -a -type f ! -wholename "*config/*" -delete
find . -type d -empty -delete
popd
DOCKER_BUILDKIT=1 docker build .

Dockerfile

FROM elixir as build
RUN mix local.hex --force && mix local.rebar --force
ADD .depscache/ /app/
RUN cd /app && mix deps.get && mix deps.compile && MIX_ENV=test mix deps.compile && MIX_ENV=prod mix deps.compile


FROM elixir
RUN mix local.hex --force && mix local.rebar --force
ADD . /app
COPY --from=build /app/_build /app/_build
COPY --from=build /app/deps /app/deps
RUN cd /app && mix compile

This could greatly benefit from caching deps during runs so stuff isn't fetched multiple times.