Every php executable produces multiple Xdebug warnings

When using PHP CLIs inside the dev container they produce multiple warnings like:

```
Xdebug: [Step Debug] Could not connect to debugging client. Tried: localhost:9000 (through xdebug.client_host/xdebug.client_port).
```

The CLIs work as intended, but the warnings make the CLI experience uncomfortable, especially when repeated 20 times after the normal output of a utility.

As I understand the image enables Xdebug functionality so user's own PHP code can be debugged with F5. I know I won't be doing that, what is the best way to disable it in a dev container?

I tried to add

```json
"remoteEnv": {
"XDEBUG_CONFIG": "mode=off"
},
```

to devcontainer.json but it has no effect.