Just for the record, I came across this yesterday, same issue, etc. and this is the final working Dockerfile I came up with, in case anybody finds it useful.

FROM elixir:1.10.3 AS app_builder

ENV MIX_ENV=prod \
    LANG=C.UTF-8

# Build local package repository
RUN mix local.hex --force && \
    mix local.rebar --force

RUN mkdir /app
WORKDIR /app

# Compile dependencies
COPY mix.exs .
COPY mix.lock .
RUN mix deps.get && mix deps.compile

# Let's make sure we have node
RUN curl -sL https://deb.nodesource.com/setup_14.x | bash - && \
    apt-get install -y nodejs

# Compile assets
COPY assets ./assets
RUN npm install --prefix ./assets && \
    npm run deploy --prefix ./assets

# Now, let's go with the actual elixir code. The order matters: if we only
# change elixir code, all the above layers will be cached ~ less image build time.
COPY config ./config
COPY lib ./lib
COPY priv ./priv

# Final build step: digest static assets and generate the release
RUN mix phx.digest && mix release

FROM debian:buster-slim AS app

ENV LANG=C.UTF-8

RUN apt-get update && apt-get install -y openssl

RUN useradd --create-home app
WORKDIR /home/app
COPY --from=app_builder /app/_build .
RUN chown -R app: ./prod
USER app

CMD ["./prod/rel/cookbook/bin/cookbook", "start"]

Please also add deps to your .dockerignoe. Some packages create artifacts in there

Also, I can’t see where you define base, but you reference it. Just from looking at it, this wont actually work.