title: Unknown command in django core managment
body: <p>I use Docker, Python and Django in my TDD project. When I run dcoker command on console:</p>
  
  <pre><code>docker-compose run app sh -c "python manage.py test"
  </code></pre>
  
  <p>The get error with message:</p>
  
  <pre><code>Starting recipe-app-api_db_1 ... done
  Creating test database for alias 'default'...
  System check identified no issues (0 silenced).
  ...EE....
  ======================================================================
  ERROR: test_wait_for_db (core.tests.test_commands.CommandsTestCase)
  Test waiting for db
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 103, in call_command
      app_name = get_commands()[command_name]
  KeyError: 'wait_for_db'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/usr/local/lib/python3.8/unittest/mock.py", line 1348, in patched
      return func(*newargs, **newkeywargs)
    File "/app/core/tests/test_commands.py", line 24, in test_wait_for_db
      call_command('wait_for_db')
    File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 105, in call_command
      raise CommandError("Unknown command: %r" % command_name)
  django.core.management.base.CommandError: Unknown command: 'wait_for_db'
  
  ======================================================================
  ERROR: test_wait_for_db_ready (core.tests.test_commands.CommandsTestCase)
  Test waiting for db when db is available
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 103, in call_command
      app_name = get_commands()[command_name]
  KeyError: 'wait_for_db'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/app/core/tests/test_commands.py", line 15, in test_wait_for_db_ready
      call_command('wait_for_db')
    File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 105, in call_command
      raise CommandError("Unknown command: %r" % command_name)
  django.core.management.base.CommandError: Unknown command: 'wait_for_db'
  
  ----------------------------------------------------------------------
  Ran 9 tests in 5.529s
  
  FAILED (errors=2)
  Destroying test database for alias 'default'...
  </code></pre>
  
  <p><strong>Source code of files</strong></p>
  
  <p>File <code>app/tests/test_commands.py</code>:</p>
  
  <pre><code>from unittest.mock import patch
  from django.core.management import call_command
  from django.db.utils import OperationalError
  from django.test import TestCase
  
  class CommandsTestCase(TestCase):
  
      def test_wait_for_db_ready(self):
          """Test waiting for db when db is available"""
  
          with patch('django.db.utils.ConnectionHandler.__getitem__') as gi:
              gi.return_value = True
              call_command('wait_for_db')
              self.assertEqual(gi.call_count, 1)
  
      @patch('time.sleep', return_value=None)
      def test_wait_for_db(self, ts):
          """Test waiting for db"""
  
          with patch('django.db.utils.ConnectionHandler.__getitem__') as gi:
              gi.side_effect = [OperationalError] * 5 + [True]
              call_command('wait_for_db')
              self.assertEqual(gi.call_count, 6)
  </code></pre>
  
  <p>File <code>app/core/managment/commands/wait_for_db.py</code>:</p>
  
  <pre><code>import time
  from django.db import connection
  from django.db.utils import OperationalError
  from django.core.management.base import BaseCommand
  
  class Command(BaseCommand):
      """Django command that waits for database to be available"""
  
      def handle(self, *args, **options):
          """Handle the command"""
          self.stdout.write('Waiting for database...')
          db_conn = None
          while not db_conn:
              try:
                  connection.ensure_connection()
                  db_conn = True
              except OperationalError:
                  self.stdout.write('Database unavailable, waiting 1 second...')
                  time.sleep(1)
  
          self.stdout.write(self.style.SUCCESS('Database available!'))
  </code></pre>
  
  <p>Why docker can't found my <code>wait_for_db</code> command file?</p>