Fixes: 1) Fix shebangs in Docker entry-point scripts to use `/usr/bin/bash`.
Extra: 1) Refactor slug fields in `core` migrations for `Brand`, `Category`, and `Product`; 2) Cleanup and simplify admin code in `core/admin.py`.
Updated the API version from 2.5.0 to 2.6.0 in the DRF settings for consistency across configurations. Reformatted migration files for better readability, ensuring consistency in code style and improving maintainability. These changes do not alter functionality but enhance clarity and documentation.
