Corepack asks if we want to install pnpm when starting the image

```bash
$ docker run --rm -it plone/plone-frontend:latest
! Corepack is about to download https://registry.npmjs.org/pnpm/-/pnpm-9.1.1.tgz
? Do you want to continue? [Y/n]
```

This is because `pnpm` expects to download the exact version required by Volto. This is bad when we want to automatically start the image.