Recently got a phoenix app running with heroku containers so I thought it would be helpful to others to see how it works. I'll be using a dummy app called bob_ross which just links out to a bunch of existing Bob Ross youtube videos.
NOTE: this is using elixir 1.11 so might not work with your version

Configs

Dockerfiles and What Not

The first step is getting your Dockerfile ready to deploy. Thankfully the folks over at google posted a nice sample dockerfile that can be easily moulded to work with heroku. Using this as a base we can easily modify it to work for heroku.

Step 1 is to change the build step to use a specific version. You rarely want to use the latest tag on any Dockerfile because updated things might break your code.

FROM elixir:1.11-alpine

Now we want to update the build step because phoenix already includes a deploy command:

RUN cd ${phoenix_subdir}/assets \
  && npm install \
  && npm run deploy \ # Line changed from raw webpack
  && cd .. \
  && mix phx.digest

The last part is to drop all the gcloud specific code in the runtime container and add a user. You'll want to add your own user because by default docker uses root which can raise security issues.

# Only set this env var
ENV REPLACE_OS_VARS=true

# 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 $PORT variable so we need to pass it to our app's start
CMD PORT=$PORT exec /opt/app/bin/start_server start 

You should be left with a dockerfile that looks something like this (make sure you change the app_name arg from bob_ross to your app)

# https://cloud.google.com/community/tutorials/elixir-phoenix-on-kubernetes-google-container-engine
# Build time container
FROM elixir:1.11-alpine

ARG app_name=bob_ross
ARG phoenix_subdir=.
ARG build_env=prod
ENV MIX_ENV=${build_env} TERM=xterm

RUN apk update \
  && apk --no-cache --update add nodejs nodejs-npm \
  && mix local.rebar --force \
  && mix local.hex --force

RUN mkdir /app
COPY . /app
WORKDIR /app

RUN mix do deps.get, compile
RUN cd ${phoenix_subdir}/assets \
  && npm install \
  && npm run deploy \
  && cd .. \
  && mix phx.digest
RUN mix release ${app_name} \
  && mv _build/${build_env}/rel/${app_name} /opt/release \
  && mv /opt/release/bin/${app_name} /opt/release/bin/start_server

# Runtime container
FROM alpine:latest
RUN apk update \
  && apk --no-cache --update add bash ca-certificates openssl-dev \
  && mkdir -p /usr/local/bin

ENV REPLACE_OS_VARS=true

# 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 exec /opt/app/bin/start_server start

I also used this .dockerignore so the builds were faster and didn't have unnecessary files in them

/_build/
/assets/node_modules/
/deps/
/doc/
/priv/static/
/test/
/tmp/
.dockerignore
Dockerfile

With that dockerfile you could try to deploy but it won't work because there's other code changes you need to make.

Config Changes

If you're coming from a slightly older elixir version you'll need to update how you pull in Config since it will error out in releases.

Start by changing references of use Mix.Config to import Config in all your configuration files. The former way of instantiating configs is deprecated now.

Next, you'll want to rename you prod.secret.exs file to releases.exs. You'll also need to remove the last line of prod.exs that imports the secret config. While you're in the prod.exs update your Endpoint configuration and strip out the line url: [host: "example.com", port: 80],. We can do this in our releases.exs so it pulls from the environment variables properly

In releases.exs we'll want to change a couple things. First if you haven't already uncomment the ssl: true line under the Repo config if you're using heroku postgres so it will work properly. Now change the Endpoint configuration to the following (using your app name, not BobRoss again):

config :bob_ross, BobRossWeb.Endpoint,
  url: [host: "bobrs.herokuapp.com", port: String.to_integer(System.fetch_env!("PORT"))],
  check_origin: ["//bobrs.herokuapp.com"],
  http: [
    port: String.to_integer(System.fetch_env!("PORT")),
    transport_options: [socket_opts: []]
  ],
  server: true,
  code_reloader: false,
  secret_key_base: secret_key_base

One notable thing that I had to do here was empty the socket_opts list. Whenever I passed the default :inet6 it would throw weird exceptions. Also make sure that any references to "PORT" throw errors if it isn't set. Your app will fail to boot and it's easier to diagnose if it raises.

You'll also need to make sure you put any static endpoint configuration in prod.exs. Otherwise, you'll get weird exceptions like ** (ArgumentError) expected these options to be unchanged from compile time: [:force_ssl]

The last configuration change I had to make was in mix.exs. You'll need to add a releases list with your app so mix release {app_name} works.

So inside mix.exs in the project function we need to add:

releases: [
   bob_ross: [
     include_executables_for: [:unix],
     applications: [runtime_tools: :permanent]
   ]
 ]

Make sure the key in the list i.e. "bob_ross:" matches your app name otherwise you'll get an error that it's not a release. The valid arguments you can pass to it can be found in the hexdocs for mix release

My full mix.exs project function looks like this

defmodule BobRoss.MixProject do
  use Mix.Project

  def project do
    [
      app: :bob_ross,
      version: "0.1.0",
      elixir: "~> 1.7",
      elixirc_paths: elixirc_paths(Mix.env()),
      compilers: [:phoenix, :gettext] ++ Mix.compilers(),
      start_permanent: Mix.env() == :prod,
      aliases: aliases(),
      deps: deps(),
      releases: [
        bob_ross: [
          include_executables_for: [:unix],
          applications: [runtime_tools: :permanent]
        ]
      ]
    ]
  end

  # Rest omitted
  ...
end

Heroku Application config

On heroku you'll need to add your SECRET_KEY_BASE variable under settings or via CLI so the app doesn't crash (mix phx.gen.secret) and also add a postgres DB if you're using that so you get the DATABASE_URL variable.

Deployment

CLI Commands

Once you have the dockerfile and config changes done you can run the app locally to see if it starts up. Make sure you have the environment variables set ($DATABASE_URL, $SECRET_KEY_BASE)

$ docker build -t <your app> .
$ docker run -p 4000:4000 -e POOL_SIZE=2 -e PORT=4000 -e DATABASE_URL=$DATABASE_URL -e SECRET_KEY_BASE=$SECRET_KEY_BASE <your app>

And if that goes well (app loads on localhost:4000) you can deploy it to heroku with their cli. Note that you will need to login to their container service before pushing heroku container:login

heroku container:push web -a <your app>
heroku container:release web -a <your app>

Running Migrations and Seeds

This isn't directly related to deployment but if you need to run a one off task via the cli on heroku it is not as easy with mix releases.

First you'll need to wrap your commands in a mix task or module that can be run through eval. For example this is my release.ex file

defmodule BobRoss.Release do
  @app :bob_ross

  @doc """
  bin/start_server eval 'BobRoss.Release.migrate()'
  """
  def migrate do
    for repo <- repos() do
      {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :up, all: true))
    end
  end

  @doc """
  bin/start_server eval 'BobRoss.Release.rollback(repo, version)'
  """
  def rollback(repo, version) do
    {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :down, to: version))
  end

  @doc """
  bin/start_server eval 'BobRoss.Release.seed()'
  """
  def seed() do
    filename = Application.app_dir(:bob_ross, "priv/repo/seeds.exs")

    for repo <- repos() do
      {:ok, _, _} =
        Ecto.Migrator.with_repo(repo, fn _repo ->
          if File.regular?(filename) do
            {:ok, Code.eval_file(filename)}
          else
            {:error, "Seeds file not found."}
          end
        end)
    end
  end

  defp start_minimal() do
    Application.ensure_all_started(:ssl)
    Application.load(@app)
  end

  defp repos do
    start_minimal()
    Application.fetch_env!(@app, :ecto_repos)
  end
end

That will let me run heroku run bash or I could run these commands directly heroku run "bin/start_server eval 'BobRoss.Release.migrate()'" and that would run the migrations on my dyno.

I think that's everything I had to do if there's any typos or misconfiguration let me know!