Jammy 24.0.5 version doesn't enable BuildKit any more

[Andreas Schildbach]

I have got

```
"features": { "buildkit": true }
```

in my `/etc/docker/daemon.json` since quite a while and it worked fine with the 20.x version from the Ubuntu Jammy repositories.

The 24.x broke this, it claims BuildKit is not enabled and refused to build:

```
the --mount option requires BuildKit. Refer to https://docs.docker.com/go/buildkit/ to learn how to build images with BuildKit enabled
```

Or

```
unknown flag: --output
```
(`--output` is a feature of BuildKit)

Restarting the Docker daemon doesn't help.