enable/disable xdebug on the fly

xdebug is slowing down the dev environment quite significantly, which is a nuisance when not developing on the API code itself.

I tried to configure xdebug in a way that it can be enabled on request only (e.g. `start_with_request=trigger`), but somehow didn't succeed.

The only thing that worked for me was to disable xdebug completely from `docker-compose.yml`:
```yaml
# XDEBUG_MODE: develop,debug,coverage
XDEBUG_MODE: "off"
```

Which is of course not ideal because:
- docker-compose down / up is needed to enable or disable
- my docker-compose.yml is dirty and I need to take care not to commit it accidentally

Anyone having more success with a better method?