add deploy to heroku

Dockerfile

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+# https://cloud.google.com/community/tutorials/elixir-phoenix-on-kubernetes-google-container-engine
+# Build time container
+ARG ALPINE_VERSION=3.15
+
+FROM elixir:1.13.3-alpine AS builder
+
+ARG app_name=farside
+ARG subdir=.
+ARG build_env=prod
+ARG SOURCE_VERSION
+ARG COMMIT=${SOURCE_VERSION}
+ARG MIX_ENV=prod
+
+ENV APP_NAME=${app_name} \
+    MIX_ENV=${MIX_ENV} \
+    COMMIT=${COMMIT}
+
+ENV REPLACE_OS_VARS=true
+ENV MIX_ENV=${build_env} TERM=xterm
+
+RUN set -ex && \
+    apk update && \
+    apk upgrade --no-cache && \
+    apk add --no-cache \
+    git \
+    curl \
+    nodejs \
+    npm \
+    python3 \
+    build-base && \
+    mix local.rebar --force && \
+    mix local.hex --force && \
+    mkdir -p /opt/built
+
+RUN mkdir /app
+COPY . /app
+WORKDIR /app
+
+RUN mix do deps.get, compile
+RUN cd ${subdir}/assets \
+  && npm install \
+  && npm run deploy \
+  && cd .. \
+RUN mix release ${app_name} \
+  && mv _build/${build_env}/rel/${app_name} /opt/release \
+  && mv /opt/release/bin/${app_name} /opt/release/bin/server
+
+# Runtime container
+FROM alpine:${ALPINE_VERSION}est
+
+RUN set -ex && \
+    apk update && \
+    apk add --no-cache \
+    bash \
+    curl \
+    gcc \
+    nodejs-current \
+    libstdc++ \
+    openssl-dev && \
+    curl https://raw.githubusercontent.com/eficode/wait-for/f71f8199a0dd95953752fb5d3f76f79ced16d47d/wait-for -o /usr/local/bin/wait-for && \
+    chmod +x /usr/local/bin/wait-for
+
+# For local dev, heroku will ignore this
+EXPOSE $PORT
+
+WORKDIR /opt/app
+COPY --from=0 /opt/release .
+RUN addgroup -S elixir && adduser -H -D -S -G elixir elixir
+RUN chown -R elixir:elixir /opt/app
+USER elixir
+
+# Heroku sets magical $PORT variable
+CMD PORT=$PORT