build: change application port from 8081 to 8000

Update Dockerfile to use port 8000 instead of 8081 for the Django
application. This change affects both the EXPOSE directive and the
Gunicorn binding configuration to standardize on the conventional
default port for web applications.

dockerfile

@@ -24,7 +24,7 @@ COPY . .
 RUN python manage.py collectstatic --noinput
 
 # Expose port
-EXPOSE 8081
+EXPOSE 8000
 
 # Run migrations and start Gunicorn
-CMD sh -c "python manage.py migrate && gunicorn booking_system.wsgi:application --bind 0.0.0.0:8081 --workers 3"
+CMD sh -c "python manage.py migrate && gunicorn booking_system.wsgi:application --bind 0.0.0.0:8000 --workers 3"