chore(docker): change application port from 8080 to 8081

Update the Dockerfile to use port 8081 instead of 8080 for the application.
This includes:
- Modified EXPOSE directive to port 8081
- Updated Gunicorn bind address to 0.0.0.0:8081

Ensures consistency between the exposed port and the actual port the
application binds to when running inside the container.

dockerfile

@@ -24,7 +24,7 @@ COPY . .
 RUN python manage.py collectstatic --noinput
 
 # Expose port
-EXPOSE 8080
+EXPOSE 8081
 
 # Run migrations and start Gunicorn
-CMD sh -c "python manage.py migrate && gunicorn booking_system.wsgi:application --bind 0.0.0.0:8080 --workers 3"
+CMD sh -c "python manage.py migrate && gunicorn booking_system.wsgi:application --bind 0.0.0.0:8081 --workers 3"