環境

開発環境はMacです。
基本的にbrewで環境整えました。

	バージョン
Ruby	2.7.2
Rails	6.1.3
PostgreSQL	12.6

手順

アプリを作成

$ rails new appname -M -d postgresql

rails new appnameでアプリ名「appname」を指定
-MでAction Mailerを組み込まないようにする
-d postgresqlで使用するDBサーバーに「PostgreSQL」を指定

参考

railsコマンドのオプション

不要なroutesを削除（Action Mailbox）

作成したアプリのroutesを確認すると、すでにいろいろなroutesが設定されています。
今回のアプリで使用しない設定は削除します。

$ bin/rails routes
                                  Prefix Verb   URI Pattern                                                                                       Controller#Action
           rails_postmark_inbound_emails POST   /rails/action_mailbox/postmark/inbound_emails(.:format)                                           action_mailbox/ingresses/postmark/inbound_emails#create
              rails_relay_inbound_emails POST   /rails/action_mailbox/relay/inbound_emails(.:format)                                              action_mailbox/ingresses/relay/inbound_emails#create
           rails_sendgrid_inbound_emails POST   /rails/action_mailbox/sendgrid/inbound_emails(.:format)                                           action_mailbox/ingresses/sendgrid/inbound_emails#create
     rails_mandrill_inbound_health_check GET    /rails/action_mailbox/mandrill/inbound_emails(.:format)                                           action_mailbox/ingresses/mandrill/inbound_emails#health_check
           rails_mandrill_inbound_emails POST   /rails/action_mailbox/mandrill/inbound_emails(.:format)                                           action_mailbox/ingresses/mandrill/inbound_emails#create
            rails_mailgun_inbound_emails POST   /rails/action_mailbox/mailgun/inbound_emails/mime(.:format)                                       action_mailbox/ingresses/mailgun/inbound_emails#create
          rails_conductor_inbound_emails GET    /rails/conductor/action_mailbox/inbound_emails(.:format)                                          rails/conductor/action_mailbox/inbound_emails#index
                                         POST   /rails/conductor/action_mailbox/inbound_emails(.:format)                                          rails/conductor/action_mailbox/inbound_emails#create
       new_rails_conductor_inbound_email GET    /rails/conductor/action_mailbox/inbound_emails/new(.:format)                                      rails/conductor/action_mailbox/inbound_emails#new
      edit_rails_conductor_inbound_email GET    /rails/conductor/action_mailbox/inbound_emails/:id/edit(.:format)                                 rails/conductor/action_mailbox/inbound_emails#edit
           rails_conductor_inbound_email GET    /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                                      rails/conductor/action_mailbox/inbound_emails#show
                                         PATCH  /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                                      rails/conductor/action_mailbox/inbound_emails#update
                                         PUT    /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                                      rails/conductor/action_mailbox/inbound_emails#update
                                         DELETE /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                                      rails/conductor/action_mailbox/inbound_emails#destroy
new_rails_conductor_inbound_email_source GET    /rails/conductor/action_mailbox/inbound_emails/sources/new(.:format)                              rails/conductor/action_mailbox/inbound_emails/sources#new
   rails_conductor_inbound_email_sources POST   /rails/conductor/action_mailbox/inbound_emails/sources(.:format)                                  rails/conductor/action_mailbox/inbound_emails/sources#create
   rails_conductor_inbound_email_reroute POST   /rails/conductor/action_mailbox/:inbound_email_id/reroute(.:format)                               rails/conductor/action_mailbox/reroutes#create
                      rails_service_blob GET    /rails/active_storage/blobs/redirect/:signed_id/*filename(.:format)                               active_storage/blobs/redirect#show
                rails_service_blob_proxy GET    /rails/active_storage/blobs/proxy/:signed_id/*filename(.:format)                                  active_storage/blobs/proxy#show
                                         GET    /rails/active_storage/blobs/:signed_id/*filename(.:format)                                        active_storage/blobs/redirect#show
               rails_blob_representation GET    /rails/active_storage/representations/redirect/:signed_blob_id/:variation_key/*filename(.:format) active_storage/representations/redirect#show
         rails_blob_representation_proxy GET    /rails/active_storage/representations/proxy/:signed_blob_id/:variation_key/*filename(.:format)    active_storage/representations/proxy#show
                                         GET    /rails/active_storage/representations/:signed_blob_id/:variation_key/*filename(.:format)          active_storage/representations/redirect#show
                      rails_disk_service GET    /rails/active_storage/disk/:encoded_key/*filename(.:format)                                       active_storage/disk#show
               update_rails_disk_service PUT    /rails/active_storage/disk/:encoded_token(.:format)                                               active_storage/disk#update
                    rails_direct_uploads POST   /rails/active_storage/direct_uploads(.:format)                                                    active_storage/direct_uploads#create

routesのパスを見たところ、Active MailboxとActive Storageの設定が入っているようです。
今回はAction Mailのみ削除します。
※ Active Storageは実際に作成しているアプリはGoogle Cloud Storageを使用する予定なので残しておきます

設定を削除

config/application.rb

require "rails"
# Pick the frameworks you want:
require "active_model/railtie"
require "active_job/railtie"
require "active_record/railtie"
require "active_storage/engine"
require "action_controller/railtie"
# require "action_mailer/railtie" 
# ↓Action Mailboxの設定をコメントアウト↓ 
# require "action_mailbox/engine"
require "action_text/engine"
require "action_view/railtie"
require "action_cable/engine"
require "sprockets/railtie"
require "rails/test_unit/railtie"

routesの確認

$ bin/rails routes
                         Prefix Verb URI Pattern                                                                                       Controller#Action
             rails_service_blob GET  /rails/active_storage/blobs/redirect/:signed_id/*filename(.:format)                               active_storage/blobs/redirect#show
       rails_service_blob_proxy GET  /rails/active_storage/blobs/proxy/:signed_id/*filename(.:format)                                  active_storage/blobs/proxy#show
                                GET  /rails/active_storage/blobs/:signed_id/*filename(.:format)                                        active_storage/blobs/redirect#show
      rails_blob_representation GET  /rails/active_storage/representations/redirect/:signed_blob_id/:variation_key/*filename(.:format) active_storage/representations/redirect#show
rails_blob_representation_proxy GET  /rails/active_storage/representations/proxy/:signed_blob_id/:variation_key/*filename(.:format)    active_storage/representations/proxy#show
                                GET  /rails/active_storage/representations/:signed_blob_id/:variation_key/*filename(.:format)          active_storage/representations/redirect#show
             rails_disk_service GET  /rails/active_storage/disk/:encoded_key/*filename(.:format)                                       active_storage/disk#show
      update_rails_disk_service PUT  /rails/active_storage/disk/:encoded_token(.:format)                                               active_storage/disk#update
           rails_direct_uploads POST /rails/active_storage/direct_uploads(.:format)                                                    active_storage/direct_uploads#create

参考

補足
ちなみにActive Storageの設定削除はrails newオプションでできるそうです。

PostgreSQLの接続設定

PostgreSQLのインストールと
config/detabase.ymlの設定をしてください。

Tailwind CSSの設定

Tailwind CSSのgemを追加

Gemfileに追記

Gemfile

gem 'tailwindcss-rails'

インストール

$ bin/bundle install
$ bin/rails tailwindcss:install

参考

tailwindcss-railsのGithub