Fix containerd (<1.7) configuration for insecure registries (#9207)

For the following configuration

```
    containerd_insecure_registries:
      docker.io:
        - dockerhubcache.example.com
```

the rendered /etc/containerd/config.toml contains

```
        [plugins."io.containerd.grpc.v1.cri".registry.configs."docker.io".tls]
          insecure_skip_verify = true
```

but it needs to be

```
        [plugins."io.containerd.grpc.v1.cri".registry.configs."dockerhubcache.example.com".tls]
          insecure_skip_verify = true
```