Developer mode workflow

### Preconditions

1. Use Magento Cloud Docker in developer mode

### Steps to reproduce

1. Follow steps in https://devdocs.magento.com/cloud/docker/docker-mode-developer.html to use cloud-docker in dev mode

### Expected result

1. Developers shouldn't have to run `docker-compose run --rm deploy magento-command deploy:mode:set developer` every time after cloud-deploy when docker-compose file was generated for developer mode. This sets the flag MAGE_MODE=developer. This should be used in cloud-deploy scenario to set the mode as developer instead of production, if present. I don't understand why we can't use that flag to set the deploy mode. Or another flag in probably `.docker/config.env`, in case the MAGE_MODE flag is meant only for the docker containers.

### Actual result

1. We have to set the deploy:mode to `developer` after every run of cloud deploy even if build:compose was run with developer flag since cloud-deploy always sets mode to production no matter the environment mode. During development, this seems unnecessary.

Start with the Magento Cloud Docker developer-mode workflow and the generated docker-compose file. Inspect `.docker/config.env`, the `build:compose` path, and the `cloud-deploy` behavior to trace why production mode is selected. Done means a developer-mode setup retains developer mode after cloud deploy without rerunning `deploy:mode:set developer`.

Written by the indexing model from the issue text.