docker-compose.yml

web: &web
    build: .
    # command: bundle exec rails s -p 3000 -b '0.0.0.0'
    volumes:
      - .:/app
      - gem_data:/usr/local/bundle
    ports:
      - "3000:3000"
    environment:
      WEBPACKER_DEV_SERVER_HOST: webpacker
      WEBPACKER_DEV_SERVER_PUBLIC: 0.0.0.0:3035
      TZ: Asia/Tokyo　←　こいつを記載

application.rb

module App
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 6.0
    # config.active_record.default_timezone = :local
    config.i18n.default_locale = :ja # デフォルトのlocaleを日本語(:ja)にする


    config.time_zone = 'Asia/Tokyo'　←　追加
    config.active_record.default_timezone = :local ←　追加


    config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.{rb,yml}').to_s]

    initializer(:remove_action_mailbox_and_activestorage_routes, after: :add_routing_paths) { |app|
      app.routes_reloader.paths.delete_if {|path| path =~ /activestorage/}
      app.routes_reloader.paths.delete_if {|path| path =~ /actionmailbox/ }
    }
    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration can go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded after loading
    # the framework and any gems in your application.
    config.paths.add 'lib', eager_load: true
  end
end

これだけで日本時間に変更可能。
＃dockerfileは書き換えたら docker-compose down、docker-compose upの手順踏まないと反映されません。