Add pdo env to php images if they are depended

[prolic]

Add pdo env to php images if they are depended

[codeliner]

We should keep it simple for now. The skeleton should help developers to get started. Everything that makes the configuration more complex than it should can be moved to documentation. In a production set up you may connect to different database anyway. You have to modify the Docker configuration for production but you can start development without pain. Simplicity is the main goal of this skeleton. Please keep that in mind.

[bweston92]

@sandrokeil environment should be fine for getting peoples feet wet. If you wanted to go to the extreme for development purposes we could add Vault using leases and audits. I use environment variables in docker-compose my development as then it is easier to manage.

[sandrokeil]

We should not write the credentials in the docker-compose.yml file. We should rely on environment variables and the user have to ensure that they are set. This is the production ready way.

Another way is to use Docker secrets. But I guess, the user input for credentials can also be an env variable or a file?