How to create/migrate Elixir Phoenix Docker image on Heroku?

I am trying to figure out how to run Elixir Phoenix on Heroku using Docker. I am pretty much using this Dockerfile: (https://github.com/jpiepkow/phoenix-docker/blob/master/Dockerfile)

# ---- Build Base Stage ----
FROM elixir:1.9.1-alpine AS app_builder
RUN apk add --no-cache=true 
    gcc 
    g++ 
    git 
    make 
    musl-dev
RUN mix do local.hex --force, local.rebar --force   

# ---- Build Deps Stage ----
FROM app_builder as deps
COPY mix.exs mix.lock ./
ARG MIX_ENV=prod
ENV MIX_ENV=$MIX_ENV
RUN mix do deps.get --only=$MIX_ENV, deps.compile

# ---- Build Release Stage ----
FROM deps as releaser
RUN echo $MIX_ENV
COPY config ./config
COPY lib ./lib
COPY priv ./priv
RUN mix release && 
    cat mix.exs | grep app: | sed -e 's/ app: ://' | tr ',' ' ' | sed 's/ //g' > app_name.txt

# ---- Final Image Stage ----
FROM alpine:3.9 as app
RUN apk add --no-cache bash libstdc++ openssl
ENV CMD=start
COPY --from=releaser ./_build .
COPY --from=releaser ./app_name.txt ./app_name.txt
CMD ["sh","-c","./prod/rel/$(cat ./app_name.txt)/bin/$(cat ./app_name.txt) $CMD"]

I have pushed to Heroku and the app is running but when I try to using database things that’s when it blows up. The logs say the database needs to be migrated, which makes sense since I haven’t done it. But now I realize I’m not sure how to do that when mix is not available and I’m using Docker.

Does anyone know how to create and migrate postgres heroku when deployed with Docker?

Source: StackOverflow