Dockerfile

FROM hexpm/elixir:1.12.0-erlang-24.0-alpine-3.13.3 AS build

# install build dependencies
RUN apk add --no-cache build-base npm git python3

# prepare build dir
WORKDIR /app

# install hex + rebar
RUN mix local.hex --force && \
    mix local.rebar --force

# set build ENV
ENV MIX_ENV=prod

# install mix dependencies
COPY mix.exs mix.lock ./
COPY config config
RUN mix do deps.get, deps.compile

# # build assets
# COPY assets/package.json assets/package-lock.json ./assets/
# RUN npm --prefix ./assets ci --progress=false --no-audit --loglevel=error

COPY priv priv
# COPY assets assets
# RUN npm run --prefix ./assets deploy
RUN mix phx.digest

# compile and build release
COPY lib lib
# uncomment COPY if rel/ exists
# COPY rel rel
RUN mix do compile, release

# prepare release image
FROM alpine:3.13 AS app
RUN apk add --no-cache bash openssl postgresql-client ncurses-libs libstdc++

ENV MIX_ENV=prod

WORKDIR /app

COPY --from=build --chown=nobody:nobody /app/_build/prod/rel/myphoenixapp ./
COPY entrypoint.sh .

RUN chown nobody:nobody /app

USER nobody:nobody

ENV HOME=/app

CMD ["bash", "/app/entrypoint.sh"]

entrypoint.sh

#!/bin/bash
# docker entrypoint script.

# assign a default for the database_user
DB_USER=${DATABASE_USER:-postgres}

# wait until Postgres is ready
while ! pg_isready -q -h $DATABASE_HOST -p 5432 -U $DB_USER
do
  echo "$(date) - waiting for database to start"
  sleep 2
done

bin="/app/bin/myphoenixapp"
eval "$bin eval \"MyPhoenixApp.Release.migrate\""
# start the elixir application
exec "$bin" "start"

working-phoenix-app-docker.md

If you have problem deploying your phoenix app with Docker, here's a Dockerfile for you.