Fix Docker healthcheck to check app's own health endpoint

Co-authored-by: pacnpal <183241239+pacnpal@users.noreply.github.com>

healthcheck.py

@@ -5,9 +5,8 @@ import os
 
 def check_health():
     try:
-        host = os.environ.get('ADGUARD_HOST', 'localhost')
-        port = os.environ.get('ADGUARD_PORT', '8000')
-        url = f'http://{host}:{port}/health'
+        port = os.environ.get('APP_PORT', '8000')
+        url = f'http://localhost:{port}/health'
         with httpx.Client() as client:
             response = client.get(url)
             response.raise_for_status()