マイグレーションの処理でエラー

環境
Winodows 1p Pro (64bit)
Vagrant 2.2.14
Ubuntu 16.04.6 LTS
Docker 18.09.7
ruby:ruby 2.7.2p137
Herokuへリリースしてからデーターベースのマイグレーションの処理をしました。

 sudo heroku run bundle exec rake db:migrate RAILS_ENV=production -a　アプリ名

しかし以下のエラーが発生しました。

ActiveRecord::DatabaseConfigurations::InvalidConfigurationError: '{ host =>  }' is not a valid configuration. Expected '' to be a URL string or a Hash.
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/database_configurations.rb:223:in `build_db_config_from_raw_config'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/database_configurations.rb:172:in `block in build_configs'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/database_configurations.rb:168:in `each'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/database_configurations.rb:168:in `flat_map'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/database_configurations.rb:168:in `build_configs'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/database_configurations.rb:19:in `initialize'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/tasks/database_tasks.rb:168:in `new'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/tasks/database_tasks.rb:168:in `for_each'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/railties/databases.rake:34:in `block (2 levels) in <main>'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/railties/databases.rake:29:in `block in <main>'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/railties/databases.rake:9:in `<main>'
/usr/local/bundle/gems/bootsnap-1.5.1/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:77:in `load'
/usr/local/bundle/gems/bootsnap-1.5.1/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:77:in `load'
/usr/local/bundle/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:326:in `block in load'
/usr/local/bundle/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:299:in `load_dependency'
/usr/local/bundle/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:326:in `load'
/usr/local/bundle/gems/activerecord-6.1.1/lib/active_record/railtie.rb:49:in `block in <class:Railtie>'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/railtie.rb:263:in `instance_exec'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/railtie.rb:263:in `block in run_tasks_blocks'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/railtie.rb:275:in `each'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/railtie.rb:275:in `each_registered_block'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/railtie.rb:263:in `run_tasks_blocks'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/application.rb:520:in `block in run_tasks_blocks'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/engine/railties.rb:15:in `each'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/engine/railties.rb:15:in `each'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/application.rb:520:in `run_tasks_blocks'
/usr/local/bundle/gems/railties-6.1.1/lib/rails/engine.rb:464:in `load_tasks'
/app/Rakefile:6:in `<top (required)>'
/usr/local/bundle/gems/rake-13.0.3/exe/rake:27:in `<top (required)>'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `<main>'
(See full trace by running task with --trace)

似たよな症状の情報を参照しました。
Rails5.2.3から6.0.2へのアップグレード後に起動できない

database.ymlを怪しんでいますが、何がおかしいのか分かりません。
{host}がおかしいとか言っていますが・・。

database.yml

# MySQL. Versions 5.5.8 and up are supported.
#
# Install the MySQL driver
#   gem install mysql2
#
# Ensure the MySQL gem is defined in your Gemfile
#   gem 'mysql2'
#
# And be sure to use new-style password hashing:
#   https://dev.mysql.com/doc/refman/5.7/en/password-hashing.html
#
default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: root
  password: password
  host: db

development:
  <<: *default
  database: app_development

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  <<: *default
  database: app_test

# As with config/credentials.yml, you never want to store sensitive information,
# like your database password, in your source code. If your source code is
# ever seen by anyone, they now have access to your database.
#
# Instead, provide the password or a full connection URL as an environment
# variable when you boot the app. For example:
#
#   DATABASE_URL="mysql2://myuser:mypass@localhost/somedatabase"
#
# If the connection URL is provided in the special DATABASE_URL environment
# variable, Rails will automatically merge its configuration values on top of
# the values provided in this file. Alternatively, you can specify a connection
# URL environment variable explicitly:
#
#   production:
#     url: <%= ENV['MY_APP_DATABASE_URL'] %>
#
# Read https://guides.rubyonrails.org/configuring.html#configuring-a-database
# for a full overview on how database connection configuration can be specified.
production:
  <<: *default
  database: <%= ENV['APP_DATABASE'] %>
  username: <%= ENV['APP_DATABASE_USERNAME'] %>
  password: <%= ENV['APP_DATABASE_PASSWORD'] %>
  host:  <%= ENV['APP_DATABASE_HOST'] %>