Change default connection to database to use host different than docker container

- After setting up the virtual environment for the project
  (note missing instruction how to work locally with venv in `Readme.md`) and
  after running the `makemigrations --dry-run` or starting the webserver,
  application is trying to connect with hardcoded hostname `db` used from Dockerfile
- This patch allows to add multiple hostname from environment variable separed
  by delimiter (I liked `,`) and since the create type in settings.py is `list` type
  it is free to use first value from a list.
- Proof
* No patch
```
$ export DJANGO_ALLOWED_HOSTS=localhost,127.0.0.1
$ ./manage.py makemigrations --dry-run
/home/anel/mariadb/feedback-plugin-backend/env/lib/python3.8/site-packages/django/core/management/commands/makemigrations.py:105:
  RuntimeWarning: Got an error checking a consistent migration history performed for database connection 'default': (2005, "Unknown MySQL server host 'db' (-3)")
  warnings.warn(
Migrations for 'feedback_plugin':
  feedback_plugin/migrations/0002_alter_rawdata_upload_time.py
    - Alter field upload_time on rawdata
```
* With patch
```
$ ./manage.py makemigrations --dry-run
Migrations for 'feedback_plugin':
  feedback_plugin/migrations/0002_alter_rawdata_upload_time.py
    - Alter field upload_time on rawdata
```
- Also server gets started correctly
```
$ ./manage.py runserver
Performing system checks...

System check identified no issues (0 silenced).

You have 19 unapplied migration(s). Your project may not work properly until you apply the migrations for app(s): admin, auth, contenttypes, feedback_plugin, sessions.
Run 'python manage.py migrate' to apply them.
May 30, 2022 - 14:51:50
Django version 3.2.13, using settings 'feedback_plugin.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
```

Author: an3l

src/feedback_plugin/settings.py

@@ -25,7 +25,7 @@
 # SECURITY WARNING: don't run with debug turned on in production!
 DEBUG = bool(os.environ['DJANGO_DEBUG'])
 
-ALLOWED_HOSTS = [os.environ['DJANGO_ALLOWED_HOSTS']]
+ALLOWED_HOSTS = os.environ['DJANGO_ALLOWED_HOSTS'].split(',')
 
 
 # Application definition
@@ -71,15 +71,16 @@
 ]
 
 WSGI_APPLICATION = 'feedback_plugin.wsgi.application'
-
-
+host_name = 'db'
+if ALLOWED_HOSTS:
+    host_name = ALLOWED_HOSTS[0]
 DATABASES = {
     'default': {
         'ENGINE': 'django.db.backends.mysql',
         'NAME': os.environ['DJANGO_DB_NAME'],
         'USER': os.environ['DJANGO_DB_USER_NAME'],
         'PASSWORD': os.environ['DJANGO_DB_USER_PASSWORD'],
-        'HOST': 'db',
+        'HOST': host_name,
         'OPTIONS': {'charset': 'utf8',
                     'use_unicode': True},
         'TEST': {