config.exs

...
config :my_app_api, MyAppApi.Endpoint,
  url: [host: "api.%%%%.app"],
  secret_key_base: "%%%%",
  render_errors: [view: MyAppApi.ErrorView, accepts: ~w(html json)],
  pubsub: [name: MyAppApi.PubSub, adapter: Phoenix.PubSub.PG2],
  live_view: [signing_salt: "%%%%"]
  
  
...

docker-compose.yml

# https://docs.docker.com/compose/compose-file/
version: '3.7'
services:
  my_app:
    build: .
    depends_on:
      - postgres
    environment:
      DATABASE_URL: ecto://my_app:my_app@postgres/my_app
      HOSTNAME: localhost
      PORT: 80
      # mix phx.gen.secret
      SECRET_KEY_BASE: %%%%
    labels:
      - "traefik.frontend.rule=Host:api.%%%%.app"
      - "traefik.frontend.entryPoints=http,https"

  postgres:
    image: postgres:11.0-alpine
    environment:
      POSTGRES_DB: my_app
      POSTGRES_PASSWORD: %%%%
      POSTGRES_USER: my_app
    ports:
      - 15432:5432
    volumes:
      - postgres-data:/var/lib/postgresql/data

volumes:
  postgres-data: {}

networks:
  default:
    external:
name: web

docker.log

> docker logs image_id_hash

18:36:14.722 [error] #PID<0.3583.0> running MyAppApi.Endpoint (connection #PID<0.3581.0>, stream id 2) terminated
Server: api.%%%%.app:80 (http)
Request: GET /icons/apple-touch-icon.png
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Phoenix.HTML.Safe not implemented for %{error: "unexpected"}. This protocol is implemented for: Decimal, Phoenix.LiveView.Comprehension, Phoenix.LiveView.Rendered, Integer, Date, Float, Phoenix.HTML.Form, Tuple, Atom, NaiveDateTime, DateTime, List, BitString, Time
        (phoenix_html) deps/phoenix_html/lib/phoenix_html/safe.ex:1: Phoenix.HTML.Safe.impl_for!/1
        (phoenix_html) deps/phoenix_html/lib/phoenix_html/safe.ex:15: Phoenix.HTML.Safe.to_iodata/1
        (phoenix) lib/phoenix/controller.ex:729: Phoenix.Controller.__put_render__/5
        (my_app_api) lib/my_app_api/endpoint.ex:1: MyAppApi.Endpoint.instrument/4
        (phoenix) lib/phoenix/endpoint/render_errors.ex:75: Phoenix.Endpoint.RenderErrors.instrument_render_and_send/5
        (phoenix) lib/phoenix/endpoint/render_errors.ex:62: Phoenix.Endpoint.RenderErrors.__catch__/5
        (phoenix) lib/phoenix/endpoint/cowboy2_handler.ex:33: Phoenix.Endpoint.Cowboy2Handler.init/2
        (cowboy) /app/deps/cowboy/src/cowboy_handler.erl:41: :cowboy_handler.execute/2
18:36:14.727 [error] #PID<0.3584.0> running MyAppApi.Endpoint (connection #PID<0.3578.0>, stream id 3) terminated
Server: api.%%%%.app:80 (http)
Request: GET /icons/favicon-32x32.png
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Phoenix.HTML.Safe not implemented for %{error: "unexpected"}. This protocol is implemented for: Decimal, Phoenix.LiveView.Comprehension, Phoenix.LiveView.Rendered, Integer, Date, Float, Phoenix.HTML.Form, Tuple, Atom, NaiveDateTime, DateTime, List, BitString, Time
        (phoenix_html) deps/phoenix_html/lib/phoenix_html/safe.ex:1: Phoenix.HTML.Safe.impl_for!/1
        (phoenix_html) deps/phoenix_html/lib/phoenix_html/safe.ex:15: Phoenix.HTML.Safe.to_iodata/1
        (phoenix) lib/phoenix/controller.ex:729: Phoenix.Controller.__put_render__/5
        (my_app_api) lib/my_app_api/endpoint.ex:1: MyAppApi.Endpoint.instrument/4
        (phoenix) lib/phoenix/endpoint/render_errors.ex:75: Phoenix.Endpoint.RenderErrors.instrument_render_and_send/5
        (phoenix) lib/phoenix/endpoint/render_errors.ex:62: Phoenix.Endpoint.RenderErrors.__catch__/5
        (phoenix) lib/phoenix/endpoint/cowboy2_handler.ex:33: Phoenix.Endpoint.Cowboy2Handler.init/2
        (cowboy) /app/deps/cowboy/src/cowboy_handler.erl:41: :cowboy_handler.execute/2
18:36:14.843 [error] Could not check origin for Phoenix.Socket transport.

Origin of the request: https://api.%%%%.app

This happens when you are attempting a socket connection to
a different host than the one configured in your config/
files. For example, in development the host is configured
to "localhost" but you may be trying to access it from
"127.0.0.1". To fix this issue, you may either:

  1. update [url: [host: ...]] to your actual host in the
     config file for your current environment (recommended)

  2. pass the :check_origin option when configuring your
     endpoint or when configuring the transport in your
     UserSocket module, explicitly outlining which origins
     are allowed:

        check_origin: ["https://example.com",
                       "//another.com:888", "//other.com"]


18:36:15.363 [error] Could not check origin for Phoenix.Socket transport.



01:26:27.023 [error] Could not check origin for Phoenix.Socket transport.

Origin of the request: https://api.%%%%.app

This happens when you are attempting a socket connection to
a different host than the one configured in your config/
files. For example, in development the host is configured
to "localhost" but you may be trying to access it from
"127.0.0.1". To fix this issue, you may either:

  1. update [url: [host: ...]] to your actual host in the
     config file for your current environment (recommended)

  2. pass the :check_origin option when configuring your
     endpoint or when configuring the transport in your
     UserSocket module, explicitly outlining which origins
     are allowed:

        check_origin: ["https://example.com",
                       "//another.com:888", "//other.com"]


01:33:54.659 [error] Postgrex.Protocol (#PID<0.3300.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
02:33:54.662 [error] Postgrex.Protocol (#PID<0.3299.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
03:33:54.664 [error] Postgrex.Protocol (#PID<0.3300.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
04:33:54.667 [error] Postgrex.Protocol (#PID<0.3303.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
05:33:54.670 [error] Postgrex.Protocol (#PID<0.3300.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
06:33:54.673 [error] Postgrex.Protocol (#PID<0.3299.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
07:33:54.677 [error] Postgrex.Protocol (#PID<0.3303.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
08:33:54.681 [error] Postgrex.Protocol (#PID<0.3300.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
09:33:54.685 [error] Postgrex.Protocol (#PID<0.3299.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
10:33:54.689 [error] Postgrex.Protocol (#PID<0.3303.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
11:33:54.693 [error] Postgrex.Protocol (#PID<0.3300.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
12:33:54.696 [error] Postgrex.Protocol (#PID<0.3299.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
13:33:54.699 [error] Postgrex.Protocol (#PID<0.3303.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)
14:33:54.701 [error] Postgrex.Protocol (#PID<0.3300.0>) disconnected: ** (DBConnection.ConnectionError) owner #PID<0.3309.0> timed out because it owned the connection for longer than 60000ms (set via the :ownership_timeout option)

release_tasks.ex

defmodule DB.ReleaseTasks do
  @start_apps [
    :crypto,
    :ssl,
    :postgrex,
    :ecto,
    :ecto_sql
  ]

  @repos Application.get_env(:db, :ecto_repos, [])

  def migrate(_argv) do
    start_services()

    run_migrations()

    stop_services()
  end

  def seed(_argv) do
    start_services()

    run_migrations()

    run_seeds()

    stop_services()
  end

  def priv_dir(app), do: "#{:code.priv_dir(app)}"

  defp start_services do
    IO.puts("Starting dependencies..")
    # Start apps necessary for executing migrations
    Enum.each(@start_apps, &Application.ensure_all_started/1)

    # Start the Repo(s) for app
    IO.puts("Starting repos..")
    Enum.each(@repos, & &1.start_link(pool_size: 2))
  end

  defp stop_services do
    IO.puts("Success!")
    :init.stop()
  end

  defp run_migrations do
    Enum.each(@repos, &run_migrations_for/1)
  end

  defp run_migrations_for(repo) do
    app = Keyword.get(repo.config, :otp_app)
    migrations_path = priv_path_for(repo, "migrations")
    IO.puts("Running migrations for #{app} repo: #{repo} migrations_path: #{migrations_path}")
    Ecto.Migrator.run(repo, migrations_path, :up, all: true)
  end

  defp run_seeds do
    Enum.each(@repos, &run_seeds_for/1)
  end

  defp run_seeds_for(repo) do
    # Run the seed script if it exists
    seed_script = priv_path_for(repo, "seeds.exs")

    if File.exists?(seed_script) do
      IO.puts("Running seed script..")
      Code.eval_file(seed_script)
    end
  end

  defp priv_path_for(repo, filename) do
    app = Keyword.get(repo.config, :otp_app)

    repo_underscore =
      repo
      |> Module.split()
      |> List.last()
      |> Macro.underscore()

    priv_dir = "#{:code.priv_dir(app)}"

    Path.join([priv_dir, repo_underscore, filename])
  end
end