How to set TYPO3_CONTEXT in ddev

My ddev environment always runs in production mode. How to change that?

SetEnv TYPO3_CONTEXT Development

Edit 2023-04-25: There are now many ways to set environment variables that don't require any of this. You can use a .ddev/.env, or add an environment variable to web_environment, etc. See https://ddev.readthedocs.io/en/latest/users/extend/customization-extendibility/#providing-custom-environment-variables-to-a-container

The answer by @marcel is better these days.

Original answer:

There's an easier way that doesn't involve such complexity.

Create a .ddev/docker-compose.typo3.yaml with these contents:

    services:
      web:
        environment:
          - TYPO3_CONTEXT=Development

This does nothing beyond adding that environment variable in the web container. Note that the name of the docker-compose file is not important, it could be docker-compose.envstuff.yaml