This is my approach for a containerized development environment using Docker
project_folder/
|-- docker/
| |-- web/
| | |-- Dockerfile
| | |-- nginx.conf
| |
| |-- app/
| | |-- Dockerfile
| |
| |-- composer/
| | |-- Dockerfile
| |
|-- env/
| |-- mysql.env
| |
|-- src/
| |-- (Your CodeIgniter application files)
| |
|-- docker-compose.yml