Code for step 3

Dockerfile

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+# ---- Build Stage ----
+FROM erlang:21 AS app_builder
+
+# Set environment variables for building the application
+ENV MIX_ENV=prod \
+    TEST=1 \
+    LANG=C.UTF-8
+
+# Fetch the latest version of Elixir (once the 1.9 docker image is available you won't have to do this)
+RUN set -xe \
+  && ELIXIR_DOWNLOAD_URL="https://github.com/elixir-lang/elixir/archive/v1.9.0-rc.0.tar.gz" \
+  && ELIXIR_DOWNLOAD_SHA256="fa019ba18556f53bfb77840b0970afd116517764251704b55e419becb0b384cf" \
+  && curl -fSL -o elixir-src.tar.gz $ELIXIR_DOWNLOAD_URL \
+  && echo "$ELIXIR_DOWNLOAD_SHA256  elixir-src.tar.gz" | sha256sum -c - \
+  && mkdir -p /usr/local/src/elixir \
+  && tar -xzC /usr/local/src/elixir --strip-components=1 -f elixir-src.tar.gz \
+  && rm elixir-src.tar.gz \
+  && cd /usr/local/src/elixir \
+  && make install clean
+
+# Install hex and rebar
+RUN mix local.hex --force && \
+    mix local.rebar --force
+
+# Create the application build directory
+RUN mkdir /app
+WORKDIR /app
+
+# Copy over all the necessary application files and directories
+COPY config ./config
+COPY lib ./lib
+COPY priv ./priv
+COPY mix.exs .
+COPY mix.lock .
+
+# Fetch the application dependencies and build the application
+RUN mix deps.get
+RUN mix deps.compile
+RUN mix phx.digest
+RUN mix release
+
+# ---- Application Stage ----
+FROM debian:stretch AS app
+
+ENV LANG=C.UTF-8
+
+# Install openssl
+RUN apt-get update && apt-get install -y openssl
+
+# Copy over the build artifact from the previous step and create a non root user
+RUN useradd --create-home app
+WORKDIR /home/app
+COPY --from=app_builder /app/_build .
+RUN chown -R app: ./prod
+USER app
+
+# Run the Phoenix app
+CMD ["./prod/rel/docker_elixir_19_release/bin/docker_elixir_19_release", "start"]