I’m trying to cram my phoenix project into a couple of containers. I have a mysql-server running in a container that exposes 3306 (and which I’m able to connect to from CLI). I’ve built a container for my app with this

# BUILD STAGE
FROM bitwalker/alpine-elixir-phoenix:latest AS phx-builder

# Set exposed ports
ENV MIX_ENV=prod

RUN mkdir /app
WORKDIR /app

# Cache elixir deps
ADD mix.exs mix.lock ./
RUN mix do deps.get, deps.compile

# Same with npm deps
ADD assets/package.json assets/
RUN cd assets && \
  npm install

ADD . .

# Run frontend build, compile, and digest assets
RUN cd assets/ && \
  npm run deploy && \
  cd - && \
  mix do deps.get, deps.compile, phx.digest, release

# APPLICATION STAGE
FROM bitwalker/alpine-elixir:latest

COPY --from=phx-builder /app/_build .

RUN chmod +x ./prod/rel/fish/bin/fish
RUN mkdir ./prod/rel/fish/tmp
RUN chmod +x ./prod/rel/fish/bin/fish

EXPOSE 5000
ENV MIX_ENV=prod 
ENV APP_PORT=4000 
ENV COOL_TEXT="Elixir Rocks" 
ENV SECRET_KEY_BASE="Aru1gpxiZDZHqt6biIsZoblCM8/yUPLy3kG9BhgcvcKUDUv5SkPhR4q/HKhZkcV7" 
ENV POOL_SIZE=10 

CMD ["./prod/rel/fish/bin/fish", "start"]

I can do

$ docker build -f Dockerfile.build -t fish .

but when I do

$ docker run --publish 4000:4000 --env COOL_TEXT='ELIXIR ROCKS!!!!' --env SECRET_KEY_BASE=fikumdick --env APP_PORT=4000 --env PORT_SIZE=10 --env HOST=localhost fish:latest

I get this output

15:31:11.834 [info] Running FishWeb.Endpoint with cowboy 2.7.0 at :::4000 (http)
15:31:11.835 [info] Access FishWeb.Endpoint at http://example.com
15:31:13.194 [error] MyXQL.Connection (#PID<0.3290.0>) failed to connect: ** (DBConnection.ConnectionError) non-existing domain

(and I haven’t the faintest idea where ‘it’ digs up that example.com - I have zero mentioning of that in any file)

My config/config.exs looks like this:

# config/config.exs
use Mix.Config

config :fish,
  ecto_repos: [Fish.Repo]

# Configures the endpoint
config :fish, FishWeb.Endpoint,
  url: [host: {:system, "HOST"}, port: {:system, "PORT"}],
  load_from_system_env: true,
  secret_key_base: "nOPS8kJTmAZtcgOpGVo7fMrAbFCCUVySKShVewMdaudersQgSLszNnTqb36fm71I",
  render_errors: [view: FishWeb.ErrorView, accepts: ~w(html json), layout: false],
  pubsub_server: Fish.PubSub,
  live_view: [signing_salt: "i1ucCSg6"]

config :fish, :pow,
  user: Fish.Users.User,
  repo: Fish.Repo,
  extensions: [PowResetPassword],
  controller_callbacks: Pow.Extension.Phoenix.ControllerCallbacks,
  web_module: FishWeb,
  mailer_backend: FishWeb.Pow.Mailer

config :fish, :pow_assent,
  providers: [
    github: [
      client_id: "REPLACE_WITH_CLIENT_ID",
      client_secret: "REPLACE_WITH_CLIENT_SECRET",
      strategy: Assent.Strategy.Github
    ]
    # example: [
    #   client_id: "REPLACE_WITH_CLIENT_ID",
    #   site: "https://server.example.com",
    #   authorization_params: [scope: "user:read user:write"],
    #   nonce: true,
    #   strategy: Assent.Strategy.OIDC
    # ]
  ]

# Configures Elixir's Logger
config :logger, :console,
  format: "$time $metadata[$level] $message\n",
  metadata: [:request_id]

# Use Jason for JSON parsing in Phoenix
config :phoenix, :json_library, Jason

and my releases.exs looks like this

#config/releases.exs 
import Config

secret_key_base = System.fetch_env!("SECRET_KEY_BASE")
cool_text = System.fetch_env!("COOL_TEXT")
application_port = System.fetch_env!("APP_PORT")
pool_size = System.fetch_env!("POOL_SIZE")

config :fish,
  cool_text: cool_text

config :fish, Fish.Repo,
  # ssl: true,
  migration_primary_key: [name: :id, type: :binary_id],
  username: "root",
  password: "",
  database: "fish_dev",
  hostname: "fish_db",
  # url: database_url,
  pool_size: String.to_integer(pool_size || "10")

config :fish, FishWeb.Endpoint,
  http: [:inet6, port: String.to_integer(application_port)],
  secret_key_base: secret_key_base