temp build container names can be too long

`error: failed setting config [lxc.uts.name](https://lxc.uts.name/) to temp-snapshot-build-cluster-api-provider-sveltos-amd64-2544038502: setting config item for the container failed`

seen with a file with a very long layer name, e.g.:

```
build-cluster-api-provider-sveltos-amd64-2544038502:
    from:
        type: docker
...
```