Squash image layers.

### What is the problem you're trying to solve

After committing a container, the commit image will continuously grow larger. Squashing the commit image layers can effectively reduce the size of the commit image.

### Describe the solution you'd like

example commands:
```bash
# squash last 2 layers.
nerdctl squash docker.io/library/busybox:latest --from-layer 2 --tag docker.io/library/busybox:squash
```

### Additional context

ref: https://github.com/goldmann/docker-squash