If you pay attention the commands I suggested have slight differences and they come directly from the Elixir docs. You can keep them separated, but you may want to adopt the official way of doing it.
For me the Dockerfile is to build a production release, therefore I prefer to not use cache at all, I even use the --no-cache flag on the command line.
In development I use this Dockerfile:
ARG ELIXIR_VERSION
ARG OTP_VERSION
ARG ALPINE_VERSION
FROM hexpm/elixir:${ELIXIR_VERSION}-erlang-${OTP_VERSION}-alpine-${ALPINE_VERSION} as build
ARG MIX_ENV=dev
ENV MIX_ENV=${MIX_ENV}
ENV USER="developer"
ENV HOME=/home/"${USER}"
ENV APP_DIR="${HOME}/workspace"
RUN \
apk upgrade --no-cache && \
apk add \
--no-cache \
inotify-tools \
openssh-client \
build-base \
npm \
git && \
# Creates a unprivileged user to run the app
addgroup \
-g 1000 \
-S "${USER}" && \
adduser \
-s /bin/sh \
-u 1000 \
-G "${USER}" \
-h "${HOME}" \
-D "${USER}" && \
su "${USER}" sh -c "mkdir ${APP_DIR}"
# Everything from this line onwards will run in the context of the unprivileged user.
USER "${USER}"
RUN \
mix local.hex --force && \
mix local.rebar --force
ARG GIT_USER_DEPLOY_TOKEN
RUN \
# @link https://github.com/elixir-lang/elixir/issues/3422#issuecomment-388188608
# @link https://gist.github.com/Kovrinic/ea5e7123ab5c97d451804ea222ecd78a
git config --global url."https://exadra37:${GIT_USER_DEPLOY_TOKEN}@gitlab.com".insteadOf git://gitlab.com
WORKDIR "${APP_DIR}"
CMD ["sh"]
