Tailwind not working in production (no styles just plain html)

Hey folks!
I deployed phoenix 1.6.5 app with tailwindcss to fly.io.
Deploying itself went successfully, but when i visit the app, i got no style. I got just plain html pages.
In the local Machine, tailwindcss works completely fine though.

About deploying, I followed offical guide.
About Tailwindcss, I used Chris Mccord’s library.

I read docs carefully, but I have no clue how to make tailwindcss work properly in production after all.

Dockerfile is like below.

# Find eligible builder and runner images on Docker Hub. We use Ubuntu/Debian instead of
# Alpine to avoid DNS resolution issues in production.
#
# https://hub.docker.com/r/hexpm/elixir/tags?page=1&name=ubuntu
# https://hub.docker.com/_/ubuntu?tab=tags
#
#
# This file is based on these images:
#
#   - https://hub.docker.com/r/hexpm/elixir/tags - for the build image
#   - https://hub.docker.com/_/debian?tab=tags&page=1&name=bullseye-20210902-slim - for the release image
#   - https://pkgs.org/ - resource for finding needed packages
#   - Ex: hexpm/elixir:1.13.1-erlang-24.2-debian-bullseye-20210902-slim
#
ARG BUILDER_IMAGE="hexpm/elixir:1.13.1-erlang-24.2-debian-bullseye-20210902-slim"
ARG RUNNER_IMAGE="debian:bullseye-20210902-slim"

FROM ${BUILDER_IMAGE} as builder

# install build dependencies
RUN apt-get update -y && apt-get install -y build-essential git \
    && apt-get clean && rm -f /var/lib/apt/lists/*_*

# prepare build dir
WORKDIR /app

# install hex + rebar
RUN mix local.hex --force && \
    mix local.rebar --force

# set build ENV
ENV MIX_ENV="prod"

# install mix dependencies and tailwind
COPY mix.exs mix.lock ./
RUN mix deps.get --only $MIX_ENV
RUN mkdir config

# copy compile-time config files before we compile dependencies
# to ensure any relevant config change will trigger the dependencies
# to be re-compiled.
COPY config/config.exs config/${MIX_ENV}.exs config/
RUN mix deps.compile

COPY priv priv

# note: if your project uses a tool like https://purgecss.com/,
# which customizes asset compilation based on what it finds in
# your Elixir templates, you will need to move the asset compilation
# step down so that `lib` is available.
COPY assets assets

# compile assets
RUN mix assets.deploy

# Compile the release
COPY lib lib

RUN mix compile

# Changes to config/runtime.exs don't require recompiling the code
COPY config/runtime.exs config/

COPY rel rel
RUN mix release

# start a new build stage so that the final image will only contain
# the compiled release and other runtime necessities
FROM ${RUNNER_IMAGE}

RUN apt-get update -y && apt-get install -y libstdc++6 openssl libncurses5 locales \
  && apt-get clean && rm -f /var/lib/apt/lists/*_*

# Set the locale
RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen

ENV LANG en_US.UTF-8
ENV LANGUAGE en_US:en
ENV LC_ALL en_US.UTF-8

WORKDIR "/app"
RUN chown nobody /app

# Only copy the final release from the build stage
COPY --from=builder --chown=nobody:root /app/_build/prod/rel/lv18 ./

USER nobody

CMD ["/app/bin/server"]

# Appended by flyctl
ENV ECTO_IPV6 true
ENV ERL_AFLAGS "-proto_dist inet6_tcp"

fly.toml is like below.

# fly.toml file generated for divine-frog-1067 on 2022-01-12T20:44:28+09:00

app = "divine-frog-1067"

kill_signal = "SIGTERM"
kill_timeout = 5
processes = []

[deploy]
  release_command = "/app/bin/migrate"

[env]
  # PHX_HOST = "divine-frog-1067.fly.dev"
  # PORT = "8080"

[experimental]
  allowed_public_ports = []
  auto_rollback = true

[[services]]
  http_checks = []
  internal_port = 4000
  processes = ["app"]
  protocol = "tcp"
  script_checks = []

  [services.concurrency]
    hard_limit = 25
    soft_limit = 20
    type = "connections"

  [[services.ports]]
    handlers = ["http"]
    port = 80

  [[services.ports]]
    handlers = ["tls", "http"]
    port = 443

  [[services.tcp_checks]]
    grace_period = "30s"
    interval = "15s"
    restart_limit = 0
    timeout = "2s"

runtime.exs is like below.

import Config

# config/runtime.exs is executed for all environments, including
# during releases. It is executed after compilation and before the
# system starts, so it is typically used to load production configuration
# and secrets from environment variables or elsewhere. Do not define
# any compile-time configuration in here, as it won't be applied.
# The block below contains prod specific runtime configuration.
if config_env() == :prod do
  database_url =
    System.get_env("DATABASE_URL") ||
      raise """
      environment variable DATABASE_URL is missing.
      For example: ecto://USER:PASS@HOST/DATABASE
      """
  app_name =
    System.get_env("FLY_APP_NAME") ||
      raise "FLY_APP_NAME not available"

  config :lv18, Lv18.Repo,
    # ssl: true,
    socket_options: [:inet6],
    url: database_url,
    pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")

  # The secret key base is used to sign/encrypt cookies and other secrets.
  # A default value is used in config/dev.exs and config/test.exs but you
  # want to use a different value for prod and you most likely don't want
  # to check this value into version control, so we use an environment
  # variable instead.
  secret_key_base =
    System.get_env("SECRET_KEY_BASE") ||
      raise """
      environment variable SECRET_KEY_BASE is missing.
      You can generate one by calling: mix phx.gen.secret
      """

  config :lv18, Lv18Web.Endpoint,
    server: true,
    url: [host: "#{app_name}.fly.dev", port: 80],
    http: [
      # Enable IPv6 and bind on all interfaces.
      # Set it to  {0, 0, 0, 0, 0, 0, 0, 1} for local network only access.
      # See the documentation on https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html
      # for details about using IPv6 vs IPv4 and loopback vs public addresses.
      ip: {0, 0, 0, 0, 0, 0, 0, 0},
      port: String.to_integer(System.get_env("PORT") || "4000")
    ],
    secret_key_base: secret_key_base

  # ## Using releases
  #
  # If you are doing OTP releases, you need to instruct Phoenix
  # to start each relevant endpoint:
  #
  #     config :lv18, Lv18Web.Endpoint, server: true
  #
  # Then you can assemble a release by calling `mix release`.
  # See `mix help release` for more information.

  # ## Configuring the mailer
  #
  # In production you need to configure the mailer to use a different adapter.
  # Also, you may need to configure the Swoosh API client of your choice if you
  # are not using SMTP. Here is an example of the configuration:
  #
  #     config :lv18, Lv18.Mailer,
  #       adapter: Swoosh.Adapters.Mailgun,
  #       api_key: System.get_env("MAILGUN_API_KEY"),
  #       domain: System.get_env("MAILGUN_DOMAIN")
  #
  # For this example you need include a HTTP client required by Swoosh API client.
  # Swoosh supports Hackney and Finch out of the box:
  #
  #     config :swoosh, :api_client, Swoosh.ApiClient.Hackney
  #
  # See https://hexdocs.pm/swoosh/Swoosh.html#module-installation for details.
end

config.exs is like below.

import Config

# For production, don't forget to configure the url host
# to something meaningful, Phoenix uses this information
# when generating URLs.
#
# Note we also include the path to a cache manifest
# containing the digested version of static files. This
# manifest is generated by the `mix phx.digest` task,
# which you should run after static files are built and
# before starting your production server.
config :lv18, Lv18Web.Endpoint,
  url: [host: "example.com", port: 80],
  cache_static_manifest: "priv/static/cache_manifest.json"

# Do not print debug messages in production
config :logger, level: :info

# ## SSL Support
#
# To get SSL working, you will need to add the `https` key
# to the previous section and set your `:url` port to 443:
#
#     config :lv18, Lv18Web.Endpoint,
#       ...,
#       url: [host: "example.com", port: 443],
#       https: [
#         ...,
#         port: 443,
#         cipher_suite: :strong,
#         keyfile: System.get_env("SOME_APP_SSL_KEY_PATH"),
#         certfile: System.get_env("SOME_APP_SSL_CERT_PATH")
#       ]
#
# The `cipher_suite` is set to `:strong` to support only the
# latest and more secure SSL ciphers. This means old browsers
# and clients may not be supported. You can set it to
# `:compatible` for wider support.
#
# `:keyfile` and `:certfile` expect an absolute path to the key
# and cert in disk or a relative path inside priv, for example
# "priv/ssl/server.key". For all supported SSL configuration
# options, see https://hexdocs.pm/plug/Plug.SSL.html#configure/1
#
# We also recommend setting `force_ssl` in your endpoint, ensuring
# no data is ever sent via http, always redirecting to https:
#
#     config :lv18, Lv18Web.Endpoint,
#       force_ssl: [hsts: true]
#
# Check `Plug.SSL` for all available options in `force_ssl`.

Thanks!